High-temperature behavior of calcium substituted layered neodymium nickelates

Article abstract of DOI:10.1016/j.jallcom.2019.05.349

Layered lanthanide nickelates of the first order with a K₂NiF₄-type structure, including Nd₂NiO₄, are considered as prospective oxygen electrode materials for different high-temperature electrochemical devices, membrane materials and catalysts. The presence of highly-mobile over-stoichiometric oxygen in their structure, the content of which depends on the synthesis and ambient conditions, strongly influences their functional properties. In this study, the materials of the Nd_2-xCa_xNiO_4+δ (x = 0–1.0) series were obtained via a citrate-nitrate method. The Ca substitution limit was found to be equal to 30 mol %. The high-temperature structural, thermo-mechanical and electrical properties of the materials obtained were investigated in close relation with their oxygen over-stoichiometry. It was found that the absolute oxygen content decreased with Ca doping, which resulted in a transition from an orthorhombic structure (Fmmm sp. gr.) at x = 0, 0.1 to a tetragonal structure (I4/mmm sp. gr.) at x = 0.2, 0.3 and then, with further increase in Ca doping, to an orthorhombic structure (Bmab sp. gr.). Dilatometric curves for all the samples possessed high-temperature breaks corresponding to the structural changes caused by oxygen release upon heating, which was revealed in the HT-XRD and TGA-DSC experiments. There was a tendency for the thermal expansion coefficient to decrease both in the low and high temperature range with Ca doping. The minimum average TEC values were found for the samples with x = 0.4 (12.1 × 10^?6 K^?1) and with x = 0.6 (11.8 × 10^?6 K^?1) according to the HT-XRD and dilatometry data, respectively. There was a maximum at x = 0.4 (135 S cm^?1) on the concentration dependence of the total conductivity obtained in the dc four-probe measurements. Materials with a medium Ca content (0.3–0.4), possessing moderate CTE values (～12 × 10^?6 K^?1), close to those of the majority of solid-state electrolytes, and showing a high level of total conductivity, can be recommended as promising materials for various electrochemical applications.

Full text of DOI:10.1016/j.jallcom.2019.05.349

Accepted Manuscript
High-temperature behavior of calcium substituted layered neodymium nickelates
S.M. Pikalov, L.B. Vedmid', E.A. Filonova, E. Yu. Pikalova, J.G. Lyagaeva, N.A.
Danilov, A.A. Murashkina
PII:
S0925-8388(19)32041-9
DOI:
https://doi.org/10.1016/j.jallcom.2019.05.349
JALCOM 50881
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 15 October 2018
Revised Date: 8 May 2019
Accepted Date: 29 May 2019
Please cite this article as: S.M. Pikalov, L.B. Vedmid', E.A. Filonova, E.Y. Pikalova, J.G. Lyagaeva,
N.A. Danilov, A.A. Murashkina, High-temperature behavior of calcium substituted layered neodymium
nickelates, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.05.349.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
High-temperature behavior of calcium substituted layered
neodymium nickelates
S.M. Pikalova, L.B. Vedmid’a,b, E.A. Filonovab, E.Yu. Pikalovab,c*,
J.G. Lyagaevab,c , N.A. Danilovb,c, A.A. Murashkinad,e
aInstitute of Metallurgy UB RAS, 101 Amundsena st., Yekaterinburg 620016,
Russia, s.pikalov@mail.ru, elarisa100@mail.ru
bUral Federal University,19 Mira st., Yekaterinburg 620002, Russia,
elena.filonova@urfu.ru, e.pikalova@list.ru
,
cInstitute of High Temperature Electrochemistry UB RAS, 20 Academicheskaya
st., Yekaterinburg 620137, Russia, e.pikalova@list.ru, yulia.lyagaeva@ya.ru
nickdanilov7@gmail.com
,
dNorth-Western State Medical University named after I.I. Mechnikov, 47
Piskarevskij prosp, St. Petersburg 195067, Russia, murashkinaaa@mail.ru
eFSBI «Petrov Research Institute of Oncology», 68 Leningradskaya st., Pesochnу,
St. Petersburg 197758, Russia, murashkinaaa@mail.ru
*Corresponding author: elarisa100@mail.ru
,
Tel. +7 9122896922, Fax. +7 (343)
267-91-86
Abstract
Layered lanthanide nickelates of the first order with a K2NiF4-type structure,
including Nd2NiO4, are considered as prospective oxygen electrode materials for
different high-temperature electrochemical devices, membrane materials and
catalysts. The presence of highly-mobile over-stoichiometric oxygen in their
structure, the content of which depends on the synthesis and ambient conditions,
strongly influences their functional properties. In this study, the materials of the
Nd2-xCaxNiO4+δ (x=0-1.0) series were obtained via a citrate-nitrate method. The Ca
substitution limit was found to be equal to 30 mol %. The high-temperature
structural, thermo-mechanical and electrical properties of the materials obtained
were investigated in close relation with their oxygen over-stoichiometry. It was
found that the absolute oxygen content decreased with Ca doping, which resulted

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in a transition from an orthorhombic structure (Fmmm sp. gr.) at x = 0, 0.1 to a
tetragonal structure (I4/mmm sp. gr.) at x = 0.2, 0.3 and then, with further increase
in Ca doping, to an orthorhombic structure (Bmab sp. gr.). Dilatometric curves for
all the samples possessed high-temperature breaks corresponding to the structural
changes caused by oxygen release upon heating, which was revealed in the HT-
XRD and TGA-DSC experiments. There was a tendency for the thermal expansion
coefficient to decrease both in the low and high temperature range with Ca doping.
The minimum average TEC values were found for the samples with x = 0.4
(12.1×10-6 K-1) and with x = 0.6 (11.8×10-6 K-1) according to the HT-XRD and
dilatometry data, respectively. There was a maximum at x = 0.4 (135 S cm-1) on
the concentration dependence of the total conductivity obtained in the dc four-
probe measurements. Materials with a medium Ca content (0.3 - 0.4), possessing
moderate CTE values (~ 12 ×10-6 K-1), close to those of the majority of solid-state
electrolytes, and showing a high level of total conductivity, can be recommended
as promising materials for various electrochemical applications.
Keywords: Ruddlesden-Popper phase, Nd2NiO4, Ca-doping, high-temperature
behavior, oxygen electrode, ceramic membrane
1. Introduction
Lanthanide nickelates, as representatives of the Ruddlesden-Popper phases, with
the general formula Lnn+1NinO3n+1, particularly one-layered oxides (n = 1)
exhibiting the K2NiF4-type structure, have been studied with interest by the
scientific community in recent years. This interest stems from their potential use as
oxygen electrodes for solid oxide fuel cells [1, 2], electrolysers [3, 4] and
electrochemical sensors [5], as ceramic membranes for oxygen separation and
methane partial oxidation [6, 7], as catalysts and photocatalysts [8, 9] etc. The
crystal structure of Ln2NiO4+δ consists of the perovskite-type LnNiO3 and double
rock-salt-type Ln2O2 layers alternating along along the c-axis [10]. A feature of
these layered oxides is the incorporation of the over-stoichiometric oxygen δ, the

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amount of which depends on the sample’s pre-history and ambient conditions, and
strongly affects the Ln2NiO4+δ structure and functional properties. Due to the
oxygen anions incorporated on interstitial lattice sites the apical oxygen becomes
disordered and the NiO6 octahedra are partly tilted around the [110]
crystallographic direction causing a tetragonal or orthorhombic syngony with
I4/mmm or Fmmm and Bmab space groups, or the appearance of two-phase regions
[11-15].
Nd2NiO4+δ (NNO), the parent material used in this study, exhibits a δ value up to
0.25 depending on the method of synthesis, with this being the highest value in the
series of the one-layered oxides [16, 17]. The values of the coefficient of oxygen
diffusion D* and surface exchange k* for NNO (4.5×10-8 cm2 s-1 and 3.4×10-7 cm
s-1 at 700 °C in air) are comparable with those for Pr2NiO4+δ (PNO) and lanthanum
cobaltite-ferrites [16]. Moreover, NNO shows a lower level of chemical reactivity
with ZrO2 and CeO2 based electrolytes compared to La2NiO4 (LNO) [18].
As with other representatives of the Ln2NiO4+δ series, NNO is considered to be a
MIEC material with a high level of ionic conductivity due to the presence of
highly mobile interstitial oxygen in its structure [10]. However, it has only a
moderate level of electronic conductivity (50 – 70 S cm-1 at 700 °C in air), which
may affect its functional properties.
It is known that Sr substitution on the Ln-site significantly increases the level of
the electronic conductivity of Ln2NiO4+δ and also enhances oxygen mobility at the
required level of doping, which is favorable both for the effective operation of the
related electrodes and for the MIEC membrane and catalytic applications [19-24].
The average thermal expansion coefficients increase with Sr-doping, however,
even at a high doping level (x ≥ 1), they remain comparable with those of the
number of solid state electrolytes - 14.0 – 15.4×10-6 K-1 and 12.3 – 14.3×10-6 K-1
for Sr-doped La2NiO4+δ and Nd2NiO4+δ (100 – 1000°C in air) [22, 24].
The doping of NNO with other alkaline earth elements is poorly represented in the
available literature where it is mainly limited to the consideration of low-doped
systems [4, 19, 16, 25, 26]. Takeda et al. [27] have shown that Ca and Ba have a

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significantly lower solid solution limit in NNO (x = 0.6) compared to that for Sr (x
= 1.4). However, taking that into account the tolerance factor values equal to 0.87
and 0.90 for Ca and Ba at x = 0.6 , they asserted that more substitution is possible
for these systems. The authors investigated the structure and electrical properties
of Ca substituted NNO in both low and intermediate temperature ranges (70 – 700
K). However, for high-temperature applications such as SOFC, SOEC, membrane
reactors, etc., more information about the high temperature properties of these
materials is necessary. Recently, we studied the high temperature properties of the
Nd2-xCaxNiO4+δ materials, obtained by using a solution assisted solid state reaction
method in a range of Ca concentration of 0 – 15 mol. %, to validate their
applicability as oxygen electrode materials for SOFCs [26]. In this study we have
extended the synthesis of the Nd2-xCaxNiO4+δ series up to x = 1.0 in order to reveal
the Ca solubility limit and have investigated the structure and phase transitions,
oxygen content, thermal and thermo-mechanical properties, and the conductivity
behavior of the materials obtained across in a wide temperature range from 25 °C
to 850 – 1000 °C.
2. Materials and methods
2.1 Synthesis technique
Nd2-xCaxNiO4+δ (x = 0 – 1.0, NCNO01-NCNO10, notations are shown in Table 1)
powders were synthesized via a citrate-nitrate method tested on different electrode
systems [25, 28, 29] using Nd(NO3)3·6H2O (99.0 %), Ca(NO3)2·4H2O (99.9 %)
and Ni(NO3)3·6H2O (98.4 %) as starting materials. Nitrates were dissolved in
distilled water at 80°C and citric acid was added in a ratio of 2:1 to the total
amount of metal cations. A 10 vol. % ammonia solution was added in the required
amount to adjust neutral pH. The resulting solution was heated and kept at 280°C
until complete water evaporation, formation of a gel-like residue and its self-
ignition with formation of highly dispersed ash of black color. The obtained ash
was then ball-milled and calcined at 700°C, 5 h to remove organics. Final
synthesis was performed at 1050 °C, 5 h and 1150 °C, 5 h (heating-cooling rate of

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5 °/min) with intermediate activation (1 h) of powders in a planetary mill
Pulverisette-6. For the heavily doped compositions (x = 0.6 – 1.0) the synthesis
was performed in compacts at 1450 °C, 5 h.
2.2. Powders’ characterization
Room-temperature XRD was performed by means of a Shimadzu XRD-7000
diffractometer with a graphite monochromator in CuKα1 radiation in the angle
range of 23°≤2θ≤81° with 0.02° scan step and a counting time of 5 s at each point.
The XRD patterns were fitted by the Rietveld method using a FullProf Suite
software.
High-temperature XRD patterns were collected using a Rigaku “Ultima IV”
diffractometer in CoKα1 radiation in the angle range of 27°≤2θ≤97° with a
scanning speed of 2.0° min–1 and 0.02° scan step in an ambient air atmosphere in
the temperature range of 25 – 850 °C every 25 – 50 ° both in the heating and
cooling modes. The samples were equilibrated for a period of 30 min at every
specified temperature.
The microstructure of the powders was examined by a high resolution scanning
electron microscope Mira 3 LMU, Tescan.
The absolute oxygen content in the samples (4 + δ) was determined in a vacuum
circulation unit by a combined gravimetric and XRD technique [30]. Before
measurements, the samples were equilibrated on oxygen content by high-
temperature annealing in air according to the following procedure: heating with a
rate of 100 °C/h up to 1100 °C, isothermal dwelling for 10 h and slow cooling
down to 25 °C, thus providing standard conditions for comparing the oxygen
content in the samples obtained at different final synthesis temperatures. The
absolute oxygen content was defined in relation to this reference state. As-
calibrated samples were weighed and then heated up to 800 °C in 10% H2 / 90%
Ar atmosphere (logPO2 = –25.5 atm, with the gas mixture flow rate of 100 ml min-
1) with long-term dwelling at the peak temperature to ensure their complete
reduction to stable oxides and Ni. The completeness of the reduction was

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examined using post-treatment XRD analysis. This method allows the absolute
amount of oxygen contained in the original sample to be determined while taking
into account the oxygen contained in the reduction products. As the samples
undergo the heat-treatment before the experiment, it was assumed that the amount
of the gases adsorbed on the surface is not significant and the loss of mass after
reduction only applies to over-stoichiometric oxygen (δ) and Ni reduction. It is
known
that
NdꢀꢁꢂꢁꢃCaꢃNiꢄꢀꢁꢆꢅNiꢅꢇꢆOꢈꢆꢉ
is
reduced
firstly
into
NdꢀꢁꢂꢁꢃCaꢃNiꢀꢆOꢈꢁꢄ.ꢊꢂꢁꢋ.ꢊꢃ (only Ni2+) according to the reaction (1) and then
into neodymium and calcium oxides and metallic Ni0 (reaction (2)) [16]:
ꢌ
ꢍ
NdꢀꢁꢂꢁꢃCaꢃNiꢄꢀꢁꢆꢅNiꢅꢇꢆOꢈꢆꢉ ꢎꢏ NdꢀꢁꢂꢁꢃCaꢃNiꢀꢆOꢈꢁꢄ.ꢊꢂꢁꢋ.ꢊꢃ
ꢐ
ꢑ
+ 1.5x + 0.5y + δ HꢀO
(1)
+ yCaO + Ni + HꢀO.
(2)
ꢌ
ꢍ
NdꢀꢁꢂꢁꢃCaꢃNiꢀꢆOꢈꢁꢄ.ꢊꢂꢁꢋ.ꢊꢃ ꢎꢏ ꢀꢁꢂꢁꢃ NdꢀOꢇ
ꢀ
The absolute oxygen content and the average Ni oxidation states were calculated
according to these formulations.
Simultaneous thermogravimetry (TGA) and differential scanning calorimetry
(DSC) in a temperature range of 25 °C to 850 °C with a heating–cooling rate of
10°C/min were performed using a NETZSCH STA 449 F3 Jupiter thermal
analyzer. For the calculation of temperature dependences of oxygen content in the
samples, the TGA data from the second cycle were taken in two subsequent
measurements.
2.3 Measurements of the compact samples
Compact samples of a bar-shape were prepared by semidry pressing at 250 MPa
with following sintering at 1450 °C for 5 h. Relative densities of the samples,
calculated from their weight and geometric sizes, were in the range of 94 – 95 %.
Total conductivity was measured by a dc four-probe method in the temperature
range of 100 – 900 °C in a cooling mode with steps every 20 °C. Measurements

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were carried out using an automatic complex Zirconia-318 (Russia). The
systematic error of the conductivity measurement error was ~3%.
Linear thermal expansion of the compact samples was measured using a Netzsch
DIL 402 PC dilatometer in the temperature range of 100 – 1000°C in air in a
heating – cooling mode with a rate of 1.5 °C min-1 with dwelling time at the peak
temperature of 1 h. The data obtained in a cooling mode were used to evaluate the
thermal expansion coefficients (TEC).
3. Results and discussion
3.1 Structure and absolute oxygen content at room temperature
According to the XRD data the complex oxides of the Nd2-xCaxNiO4+δ (NCNO)
series were single-phase in the range of of Ca content x = 0.0 − 0.5 (Table 1). The
sample with x = 0.6 contained impurity phases such as NiO and CaO in the amount
of approximately 3 wt %. In the samples with x = 0.8 and x = 1.0, the main phase
with K2NiF4-type structure with composition of Nd1.45Ca0.55NiO4+δ coexisted with
NiO and CaO oxides in amount of which was evaluated from the main peaks’
intensity as 7 and 10 wt %, respectively. We can conclude that the Ca solubility
limit in the NNO structure is in a range of 25-30 mol % which is in good
agreement with the data obtained in [27].
An example of the power’s microstructure is show in Fig.1 S. The powders
produced via the citrate-nitrate method are quite agglomerated, however the
agglomerates have a sponge-like structure, which can be easily broken using
mechanical treatment in a planetary mill, which is favorable for the use such the
materials for the catalyst and electrode application [29]. Coarseness of the
materials increases with Ca content increasing. The compositions with Ca content
up to x= 0.4 are characterized by polydispersity, which is clearly seen at high
magnification (Fig. 2S). With further increasing the Ca content, fractional
composition becomes more uniform. The shape of the individual particles of the

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materials evolves from round (at low Ca content) to multi-faceted (at high Ca
content).
Results of the Rietveld refinement are shown in Figs. 3S – 9S (Supplementary
materials). Oxides of the NCNO series crystallized in various types of crystal
structure: NNO and NCNO01 possessed an orthorhombic symmetry with a space
group of Fmmm, NNO02 and NCNO03 crystallized in a tetragonal symmetry (sp.
gr. I4/mmm), and NNO04, NCNO05, and the main phase in NCNO06 – in an
orthorhombic one (sp.gr. Bmab). Two structural phase transitions at x 0.15 and
x 0.35 in the series, observed at room temperature, are expected to appear due to a
gradual decrease in the content of the over-stoichiometric interstitial oxygen with
increasing Ca content, which was confirmed by the gravimetric data on the
absolute oxygen content obtained in this study (Table 2). For the lightly (x = 0.1 –
0.2) doped samples the δ value was found to be in the range of 0.16 – 0.20, for the
intermediate Ca content (x = 0.3 – 0.4) it varied in the range of 0.11 – 0.13, and for
heavily doped samples (x > 0.4) δ ≤ 0.08. These findings correlate with the NNO
Ellingham diagram presented in [31], according to which the orthorhombic
symmetry in air is revealed at δ > 0.155. However, the space groups defined in
different studies were quite different: Fmmm sp.gr. was observed for NNO and
NCNO01 in this study and for NNO in [13, 32, 33]; Bmab sp. gr. was reported in
[16] for NNO and NCNO01 (δ = 0.22 and 0.20, respectively) and in [27] (δ = 0.12
and 0.10, respectively). Takeda et al. demonstrated an orthorhombic structure with
Bmab sp. gr. [27] for the samples with medium Ca content (0.1 < x ≤ 0.3, - 0.01≤ δ
≤ 0.09). Boehm et al. reported orthorhombic structure with Fmmm sp. gr. for
NCNO02 (δ = 0.10) [16]. For heavily doped samples (0.4 ≤ x ≤ 0.6, - 0.03≤ δ ≤
0.05) Takeda et al. revealed an orthorhombic structure with Fmmm sp. gr. [27].
According to the literature data, Sr- and Ba-doped NNO possess the same or lower
δ values compared to those for Ca-doped NNO (at the equal doping level) and
exhibit a tetragonal structure over the entire doping range [24, 27, 31, 32].

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A decrease in the lattice parameters and unit cell volume values in the series up to
x = 0.3 may be explained by a dimensional effect caused by the substitution of a
quantity of Ni2+VI(r = 0.69 Å [34]) ions with smaller Ni3+ ions (r = 0.56LS Å;
VI
0.60HS Å) for a charge compensation owing to Ca2+ substitution for Nd3+. The
decreasing trend for the unit cell volumes dominates over the increasing trend
caused by the substitution of Nd3+IX (r = 1.163 Å) with larger Ca2+IX ions (r = 1.18
Å). The most extreme of the concentration dependences of the NCNO lattice
parameters corresponded to x = 0.4 (Fig. 1) and was observed for the lattice
parameters’ dependences in the Pr2-xCaxNiO4+δ series at the same Ca content [35].
A similar feature was also observed for the SrLnNiO4+δ series (Ln = La, Nd, Sm,
Eu, Gd) [36] and may be explained by the transition of Ni3+ cations from a low- to
a high spin state.
Analysis of the bond lengths (Table 1) revealed compression of the perovskite-like
layers and expansion of the rock salt layers in the series with an increase in Ca
content. The length of Nd(Ca)-O2 bonds, responsible for the thickness of the rock
salt layers, increased due to the replacement of Nd3 + with larger Ca2 + cations. In
perovskite layers a decrease in the Ni-O1 bond length is observed due to the
oxidation of Ni2 + ions to smaller Ni3+ ions. The observed changes in the size of the
layers contribute to the stability of the structure (increase in the tolerance factor);
however, the ability to uptake interstitial oxygen in the structure decreases, which
is confirmed by the calculated occupation of the cation positions Occ. (Nd/Ca)
(Table 1) and the thermogravimetric studies performed in this work.
3.2 Temperature dependences of oxygen stoichiometry and structural properties
Figs. 10S-12S present examples of TGA curves, obtained from two subsequent
measurements, first derivatives of the TGA curves and relative mass loss upon
heating from 25 to 850 °C. HT-XRD data for the selected samples, collected in the
heating-cooling modes, are presented in Figs. 13S – 16S.
Temperature dependences of the absolute oxygen content (4+δ) in the samples,
calculated from the TGA data are shown in Fig. 2. In the temperature range of

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350-400 °C the oxygen content changes insignificantly and for all the samples one
can observe a horizontal plateau. The width of the plateau is defined by the
characteristic temperature (TOrel) when the sample/gas phase oxygen exchange
starts. The characteristic temperatures were defined from the first derivatives of the
mass change (Table 2). There was no direct correlation found between Ca-content
and the characteristic temperature values, which probably depend not only on the
initial oxygen content, but also on overall oxygen mobility and surface reactivity
[35, 37, 38]. However, they closely concur with “humps” on the temperature
dependence of the unit cell volumes, calculated from the HT-XRD data, which
appear in the same temperature region (Fig. 3). The bends on the TGA curves near
600 °C and 350 °C (Fig. 2, Fig. 10S) for NNO and NCNO01, respectively, which
correlate with endoeffects on the DSC curves (Fig. 4), are caused by phase
transitions from the orthorhombic structure to that of tetragonal. On the XRD
patterns, collected both in the heating and cooling modes, at these temperatures
one can observe a transition of duplets (crystallographic planes with 200 and 020
Miller indexes) to singlets (Figs. 13S, 14S). Similar transitions were observed for
the La2-xNdxNiO4+δ series, although at higher x values [39]. The temperature
dependences of the unit cell parameters calculated from the HT-XRD data also
illustrate the Fmmm→ I4/mmm transition at temperatures above 600 °C and 300
°C for NNO and NCNO01 (Figs. 5 a, b, respectively). Our findings on the NNO
structural transition with concurs closely of Boehm et al. [16], who observed the
Bmab→ I4/mmm transition in the temperature range of 550 – 600 °C.
A decrease in the temperature of the structural phase transition upon Ca doping
can be explained by a decrease in the amount of over-stoichiometric oxygen in the
NCNO01 structure as compared with the undoped NNO composition. As we stated
in part 3.1, Ca doping leads to a compression of the perovskite-like layer
(remarkable reduction of the unit cell parameters) due to a change in the ratio of
nickel ions with a different actual valence to maintain overall electroneutrality. At
the same time, Ca doping causes the expansion of the rock salt layers due to a size
effect. This results in an increase of the tolerance factor value [10, 40] for

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NCNO01 (t = 0.869) compared to NNO (t = 0.867), and reduces the ability of the
layered structure to adopt interstitial oxygen (Table 1, 2). It should be noted, that
with a decrease in the absolute oxygen content, its relative amount, released from
the samples upon heating, also decreases (Fig. 12S) and reaches its minimum at x
= 0.4. For that reason, all the samples across the entire temperature range under the
study remain over-stoichiometric.
The bends on the TGA curves and the endoeffects of low intensity on the DTA
curves for NCNO02 and NCNO03 at approximately 400 °C (Fig. 4) correlate well
with the bends on the temperature dependence of the lattice parameters (Fig. 5 c, d,
respectively) and the cell volume (Fig. 3). These features appear due to the Ni3 +
→ Ni2 + partial reduction with a gradual reduction in the oxygen content with the
temperature increasing. For NCNO04, the TOrel (Table 2) closely correlates with
the Bmab→ I4/mmm transition temperature (Fig. 6 a). There is no effect on the
DTA curve at this temperature, which indicates a decrease in the intensity of
oxygen heteroexchange. For NCNO05, the transition from the orthorhombic to the
tetragonal structure proceeds at about 100 °C (Fig. 6 b). Contrarily, for NCNO06,
the phase transition from the orthorhombic to the tetragonal takes place at a
temperature of approximately 370 °C (Fig. 6 c). Weak endoeffects on the DSC
curves in the range of 770–820 °C for all Ca-doped samples exhibiting tetragonal
structure in the high temperature range can be related to the beginning of the
oxygen vacancies’ formation in the perovskite layers [41]. The gravimetry and
thermogravimetry data were used to calculate the average oxidation state of the
nickel cations at different temperatures (Table 2). It was found that for NNO it is
close to +2.4, which contributes to the fast exchange of electrons between nickel
and oxygen. An increase in the Ca content leads to an increase in the number of
Ni3+ cations with the corresponding increasing of the Ni average oxidation state
from 2.40 (NNO) to 2.65 (NCNO06). When linearly heated to up to 850 °C, the
average oxidation state of the nickel ions in samples decreases, however this
change becomes less pronounced with doping. For instance, the values of the

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average oxidation state at room and at 850°C for NNO and NCNO01 differ by 0.2,
while for the samples with x> 0.1 such a difference is less than 0.1.
3.3 Linear thermal expansion in the high temperature range
Linear thermal expansion of the samples in the heating and cooling modes
(dilatometry method) are depicted in Figs. 7 a and 7 b, respectively. There are
“peculiarities” of the dependences, that are more pronounced during heating. The
linear slope increases in the high-temperature range, most probably due to the
variation in oxygen stoichiometry and the related partial reduction from Ni3+ to
Ni2+ with a larger cation radius. Appearance of the oxygen vacancies in the
perovskite layers in the heavily doped samples can also contribute to chemical
expansion of the samples [41]. The values of thermal expansion coefficients for
the NCNO series, calculated from the HT-XRD data (TECXRD) and dilatometry
data (TECDil), both collected in the cooling modes, are given in Table 3. One can
distinguish some visible tendencies in the TEC values’ variation: 1). The values
calculated from the HT-XRD data are lower than those from the dilatometry data.
The reason for this may be asymmetry in expansion typical for the layered
materials and different conditions for the sample equilibration; 2) TEC values in
the low temperature range are significantly lower than those in the high-
temperature range (both methods); 3) The temperatures of the low→ high
temperature transition closely correlate for both methods (except for NCNO01); 4)
TEC value decrease with an increase in Ca content except for NCNO01 (both
methods); 5) Minimal values of the average TEC were found for NCNO04, which
correlates well with the Nd(Ca)–O2 bond length (Table 1), which is the smallest
for this sample. According to the study on the effect of Sr content on the crystal
structure and electrical properties of the system La2−xSrxNiO4+δ [42], it is the Ln–
O2 bond length that is responsible for the connection of the perovskite layers and
the layers of the rock salt structure. When investigating thermal expansion of Ln2-
xCaxNiO4+δ, the minimal TEC values were also observed at x = 0.4 and 0.6 for Pr-
based system [43] and x=0.4 for La-based system [44]. Similarly, for Sr-doped

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NNO (TEC values are in the range of 12.3 – 14.3×10-6 K-1 [24]), TEC values,
obtained for the Ca-doped NNO, were sufficiently low over the entire series thus
ensuring thermo-mechanical compatibility with widely used solid electrolytes (~12
×10-6 K-1 for CeO2 and BaCeO3 based electrolytes and ~ 10×10-6 K-1 for
ZrO2,LaGaO3, BaZrO3 and LaScO3 based electrolytes).
3.4 Temperature dependences of total conductivity
Arrhenius dependences of the total electrical conductivity of the compact NCNO
samples are shown in Fig. 8. Conductivity initially increases with the calcium
content reaching the maximum at x = 0.4 (135 S cm-1 at 580 °C). This tendency is
consistent with the electroneutrality condition [26] and is caused by an increase in
the electronic conductivity due to an increase in the concentration of the mobile
charge carrier ( NiN• i ) concentration:
ꢕꢕꢖ ꢒ ••ꢖ
ꢒ
2 ꢓꢔ + ꢗꢘꢙꢚ ↔ ꢛꢜꢙꢔ + 2 ꢝꢞ
ꢖ
ꢒ
ꢖ
ꢕ
•
ꢒ
(3)
Nakamura et al [20] also observed relatively high σ values for the Nd2-xSrxNiO4+δ
((NSNO) series ( ~160 S cm-1 at 700 °C at x = 0.4) and low positive Seebeck
coefficient values, showing that the charge carries in Nd2-xSrxNiO4+δ are the
electron holes. Due to the higher substitution limit for Sr (x ~ 1.6) compared to
that for Ca (x ~ 0.6) the maximum conductivity for NSNO was registered at x =
1.2 (290 – 510 S cm-1 at 600 – 900 °C) [24]. The hole mobility, µp, evaluated
according to Eq. (4) and (5) [20] on the assumption that interstitial oxygen is
doubly charged, are presented in Table 4.
ꢡ
ꢟꢠ = ꢢꢠ,
(4)
(5)
ꢣ = ꢙꢥ ꢧ + 2ꢨ ,
ꢤ ꢐ
ꢑ
ꢦ
where e, σ, p, Vm and NA are the elementary charge, the conductivity, the hole
concentration, the molar volume and the Avogadro constant, respectively. The Vm
were taken from the HT-XRD data. The hole mobility at 100 °C for the samples up
to varies within the range of 0.05 – 0.11 cm2 V-1s-1. It increases up to x=0.3 and
then decreases. At 850 °C, the hole mobility gradually decreases from 0.34 cm2 V-

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1s-1 (NNO) to 0.19 cm2 V-1s-1 (NCNO05 and NCNO06). For the comparison, the
value of the hole mobility obtained in [20] for Nd1.8Sr0.2NiO4+δ was equal to
approximately 0.2 cm2 V-1s-1. The hole mobility in Ca-doped NNO seems to be
higher compared to that in Sr-doped NNO.
The values of the activation energy, Ea, calculated on the linear sections of the
Arrhenius dependences corresponding to the absence of oxygen release from the
samples (linear sections on the TGA curves) are shown in Table 4. It can be seen
across the samples that, as the calcium content increases from 0 up to 0.3, it is
apparent that Ea is about to remain constant, however it increases with further
substitution. Decreasing the total conductivity and hole mobility and increasing the
activation energy for the heavily doped samples can be caused, according to [45,
46], by an increase in the proportion of nickel ions Ni3+ in a high-spin state. This is
because the mobility of the localized electron holes, corresponding to the Ni3+
high-spin state, is significantly low than that of the quasi-delocalized electron
holes. The range of the Ea values of 0.07 – 0.09 eV corresponds to a hopping
motion of small-radius polarons [47-49].
With the exception of NCNO6, all the samples studied are characterized by a
change in the type of conductivity from semiconducting to quasi-metallic in the
temperature range of 500 – 600 °C. The semiconductor character of the
conductivity of samples at low temperatures proceeds according to the hopping
mechanism of small-radius polarons, as we have mentioned above. The inflection
points on the Arrhenius dependences of the electrical conductivity correspond to
the oxygen release from the samples (Fig. 2, Fig. 8S). This can be explained by the
fact that a decrease in the number of interstitial oxygen results in partial
annihilation of the holes due to reduction of Ni3+ to Ni2+ [47, 50]. Due to the larger
radius of Ni2+ cations, the Ni–O1 and Ni–O2 bond lengths will also increase,
which, in the framework of the hopping mechanism, can additionally cause the
electron holes' mobility to be reduced [38]. The NCNO06 sample exhibits a
semiconducting type of conductivity over the entire temperature range of 100 –

ACCEPTED MANUSCRIPT
900 °C. It worth nothing that similar behavior was observed for heavily doped
samples of the Pr2-xCaxNiO4+δ series (PCNO06 and PCNO07) [51].
The conductivity obtained for the NCNO series was higher than that for the LCNO
(maximum for LCNO04, 121 S/cm) [44] and comparable with that for the
PCNO04 (maximum for PCNO05, 140 S/cm) [51]. The difference in the electrical
conductivity between the series is caused by the difference in hole concentration.
The Nd and Pr-based series exhibit higher oxygen excess compared to the La-
based series in the entire range of Ca doping, and have larger hole concentration,
as hole creation is accompanied by the interstitial oxygen formation [20].
Moreover, increased amount of the interstitial oxygen is favourable for the
application of these materials as catalytic membranes for oxygen and syngas
production [52].
Conclusions
Materials of the Nd2-xCaxNiO4+δ (x=0-1.0) series were synthesized via a citrate-
nitrate method with final calcinations in air atmosphere at 1150 °C (x = 0 – 0.5)
and 1450 °C (x = 0.6 – 1.0). It was found that the Ca substitution limit was equal
to 30 mol %. High-temperature properties of the series were for the first time
investigated using TGA-DSC analysis, high temperature XRD, dilatometry and the
dc four-probe method. It was found that the absolute oxygen content decreased
with Ca doping, which resulted in a transition from an orthorhombic structure
(Fmmm sp. gr.) at x = 0, 0.1 to tetragonal one (I4/mmm sp. gr.) at x = 0.2, 0.3 and
then, with further increasing of Ca content, to an orthorhombic structure (Bmab sp.
gr.). The thermal expansion coefficients tended to decrease with Ca doping with
minimum at x = 0.4 and 0.6. Concentration dependence of total conductivity
showed an extreme behavior with a maximum at x = 0.4 (135 S cm-1). A decrease
in the charge carries’ mobility and the appearance of secondary phases in the
heavily doped samples were believed to be possible reasons for the conductivity
decreasing at x = 0.5 – 0.6. NCNO03 and NCNO04 materials, possessing

ACCEPTED MANUSCRIPT
moderate CTE value and showing a high level of total conductivity can be
recommended as a promising material for different electrochemical applications.
Acknowledgements
Investigation of oxygen stoichiometry, structure and high temperature phase
transitions were carried out in IMET UB RAS using the equipment of the Shared
Access Center “Ural-M”. Dilatometry and total conductivity measurements as well
as microstructure study were performed in IHTE UB RAS using facilities of the
Shared Access Centre “Composition of compounds”. Support of Government of
the Russian Federation (Agreement 02.A03.21.0006, Act 211) is gratefully
acknowledged. We thank Mr Andrey Farlenkov (IHTE UB RAS) for the
microstructure studies and Mr Peter Roy Orman (BA in English Language &
Linguistics VUW, New Zealand) for the manuscript proof-reading.
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51. E. Pikalova, A. Kolchugin, N. Bogdanovich, D. Medvedev, J. Lyagaeva, L.
Vedmid’, M. Ananyev, S. Plaksin, A. Farlenkov, Suitability of Pr2–xCaxNiO4+δ as
cathode materials for electrochemical devices based on oxygen ion and proton
conducting solid state electrolytes, Int. J. Hydrogen Energy 2019 In Press
https://doi.org/10.1016/j.ijhydene.2018.06.023
52. P.-M. Geffroy, M. Reichmann, T. Chartier, J.-M. Bassat, J.-C. Grenier,
Evaluating oxygen diffusion, surface exchange and oxygen semi-permeation in
Ln2NiO4+δ membranes (Ln = La, Pr and Nd), J. Membr. Sci. 451 (2014) 234–242.
https://doi.org/10.1016/j.memsci.2013.08.035

ACCEPTED MANUSCRIPT
Figure captions
Figure 1. Concentration dependences of the cells parameters of Nd2-xCaxNiO4+δ
(XRD at room temperature)
Figure 2. Temperature dependences of the absolute oxygen content (4+δ) in the
Nd2-xCaxNiO4+δ samples (gravimetry)
Figure 3. Temperature dependences of the unit cell volume of Nd2-xCaxNiO4+δ
(HT-XRD data)
Figure 4. DSC curves for the Nd2-xCaxNiO4+δ samples
Figure 5. Temperature dependences of the unit cell parameters calculated from the
HT-XRD data for Nd2NiO4+δ (a), Nd1.9Ca0.1NiO4+δ (b), Nd1.8Ca0.2NiO4+δ (c) and
Nd1.7Ca0.3NiO4+δ (d)
Figure 6. Temperature dependences of the unit cell parameters calculated from the
HT-XRD data for Nd1.6Ca0.4NiO4+δ (a), Nd1.5Ca0.5NiO4+δ (b) and Nd1.4Ca0.6NiO4+δ
(c)
Figure 7. Linear thermal expansion of the samples collected in a heating (a) and
cooling (b) modes (dilatometry method)
Figure 8. Arrhenius dependences of the total electrical conductivity of the compact
Nd2-xCaxNiO4+δ samples

ACCEPTED MANUSCRIPT
Table 1. Structural parameters of Nd2-xCaxNiO4
x
Designation
Structure
Space group
a, A
0.0
NNO
O
0.1
NCNO01
O
0.2
NCNO02
T
I4/mmm
3.8042(1)
3.8042(1)
12.3204(2)
178.300(5)
7.27
0.3
NCNO03
T
I4/mmm
3.7993(1)
3.7993(1)
12.2928(3)
177.440(5)
7.11
0.4
NCNO04
O
0.5
NCNO05
O
0.6
0.8
NCNO08
O+NiO, CaO
Bmab
5.3223(5)
5.3904(5)
12.290(1)
352.58(6)
6.65
1.0
NCNO06
O+NiO, CaO
Bmab
5.3104(2)
5.3956(2)
12.2317(5)
350.54(3)
6.61
NCNO10
O+NiO, CaO
Bmab
5.3035(3)
5.3729(3)
12.2576(7)
349.28(3)
6.58
Fmmm
Fmmm
Bmab
Bmab
5.3759(1)
5.4596(1)
12.3652(3)
362.92(1)
7.53
5.3671(2)
5.4205(2)
12.3394(4)
358.98(2)
7.42
5.3204(2)
5.3943(2)
12.2549(4)
351.72(2)
6.98
5.3076(2)
5.3625(2)
12.2772(4)
349.43(2)
6.83
b, A
c, A
V, A3
ρ, г/см3
z (Nd/Ca)
z(O2)
Occ. (Nd)/Occ. (Ca)
Boverall, A2
Nd/Ca-
0.3584(1)
0.176(1)
2.18(1)
0.52(5)
3.503(1)
0.3598(1)
0.173(1)
0.3600(1)
0.175(1)
0.3604(1)
0.175(1)
0.3610(1)
0.176(1)
0.3605(1)
0.175(1)
0.3607(1)
0.175(1)
1.96(1)/0.16(1) 1.84(1)/0.24(1) 1.74(1)/0.34(1) 1.68(1)/0.48(1) 1.57(1)/0.57(1) 1.56(1)/0.76(1)
0.52(4)
3.461(2)
0.91(4)
3.451(1)
0.74(3)
3.434(1)
0.26(4)
3.408(1)
0.46(3)
3.425(2)
1.17(3)
3.409(2)
Nd/Ca
Ni-Nd/Ca
Ni-Ni
3.208(1)
3.8310(1)
1.9155(1)
3.193(1)
3.8140(1)
1.9070(1)
3.196(1)
3.8042(1)
1.9021(1)
3.188(1)
3.7993(1)
1.8996(1)
3.190(1)
3.7883(1)
1.8942(1)
3.182(1)
3.7725(1)
1.8863(1)
3.191(1)
3.7853(1)
1.8926(1)
Ni-O1x4,
along a-axis
L, Å Ni-O2x2,
along c-axis
Nd/Ca-O1x4
Nd/Ca-O2x4
Nd/Ca-O2x1
RBr
2.17(1)
2.14(1)
2.15(1)
2.15(1)
2.16(1)
2.15(1)
2.14(1)
2.595(1)
2.721(1)
2.26(1)
2.92
2.575(1)
2.714(2)
2.30(1)
3.13
2.568(1)
2.724(1)
2.28(1)
2.47
2.560(1)
2.721(1)
2.28(1)
2.60
2.548(1)
2.735(1)
2.26(1)
4.13
2.548(1)
2.717(1)
2.28(1)
4.25
2.547(1)
2.732(2)
2.28(1)
4.80
Rf
1.79
2.05
2.00
1.93
2.71
3.52
2.81
Chi2
7.13
9.97
5.71
6.54
10.4
8.52
4.57

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Table 2. Oxygen over-stoichiometry and average Ni oxidation state in
Ni
oxidation
state
Ni
oxidation
state
4+δ
gravimetry,
25 °C, air
4+δ
TOrel,
°C
Composition
thermogravimetry
850 °C, air
25 °C, air
850 °C, air
NNO
4.20(2)
4.16(2)
4.13(2)
4.11(2)
4.08(2)
4.04(2)
4.02(2)
2.40
2.42
2.45
2.52
2.56
2.58
2.65
324
332
331
343
331
328
334
4.19(2)
4.15(2)
4.12(2)
4.10(2)
4.07(2)
4.03(2)
4.01(2)
2.21
2.27
2.37
2.43
2.51
2.51
2.60
NCNO01
NCNO02
NCNO03
NCNO04
NCNO05
NCNO06
Nd2-xCaxNiO4+δ at different temperatures in air

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Table 3. TEC values of the Nd2-xCaxNiO4+δ series calculated from the HT-XRD
(TECXRD) and dilatometry (TECDil) data
HT-XRD data
Dilatometry data
Average
TECXRD
×106, K -1
12.8(2)
Average
TECDil
Temperature
range, °C
TECXRD
Temperature
range, °C
TECDil
Sample
NNO
×106, K -1
×106, K -1
×106, K -1
14.9(1)
25-600
600-850
25-300
300-850
25-490
490-850
25-450
450-850
25-400
400-850
25-450
450-850
25-450
450-850
12.2(4)
14.5(5)
12.0(1)
13.6(2)
12.0(3)
13.9(2)
11.7(3)
13.8(2)
10.3(4)
13.2(2)
10.7(9)
14.3(4)
10.6(8)
12.7(4)
100-610
610-1000
100-570
570-1000
100-540
540-1000
25-450
450-1000
25-400
400-1000
25-450
14.0
16.4
14.2
16.3
13.6
16.1
13.7
16.2
12.1
15.6
12.0
15.5
10.5
13.3
NCNO01
NCNO02
NCNO03
NCNO04
NCNO05
NCNO06
13.2(2)
12.9(2)
12.8(2)
12.1(2)
13.6(4)
12.5(3)
15.1(2)
14.8(1)
15.1(2)
14.1(2)
14.4(1)
11.8(1)
450-1000
25-450
450-1000

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Table 4. Electrical properties of the compact Nd2-xCaxNiO4+δ samples
σmax [S cm-1]
Tmax, °C
91/600
Ea [eV]
σ [S cm-1]
µp [cm2 V-1s-1]
Composition
(100 – Tc) °C
0.074±0.003
0.069±0.002
0.070±0.002
0.078±0.002
0.077±0.002
0.080±0.002
0.095±0.004
850 °C/100 °C 850 °C/100 °C
NNO
76.88/38.81
88.83/44.85
100.16/51.91
105.64/48.84
120.03/49.55
103.05/36.13
123.58/33.28
0.34/0.09
0.31/0.10
0.27/0.11
0.23/0.09
0.22/0.08
0.19/0.06
0.19/0.05
NCNO01
NCNO02
NCNO03
NCNO04
NCNO05
NCNO06
99/560
114/540
122/520
135/580
111/640
119/640