Directed: Ortho-metalation-nucleophilic acyl substitution strategies in deep eutectic solvents: The organolithium base dictates the chemoselectivity

Article abstract of DOI:10.1039/c9cc03927a

Directed ortho metalation (DoM) or nucleophilic acyl substitution (S_NAc) can be efficiently programmed on the same aromatic carboxylic acid amide, in a choline chloride-based eutectic mixture, by simply switching the nature of the organolithium reagent. Telescoped, one-pot ortho-lithiation/Suzuki-Miyaura cross-couplings have also been demonstrated for the first time in Deep Eutectic Solvents.

Full text of DOI:10.1039/c9cc03927a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C9CC03927A
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ABO3 and A1-xCxB1-yDy(O1-zEz)3: Review of Experimental
Optimisation of Thin Film Perovskites by High-Throughput
Evaporative Physical Vapour Deposition.
8Received 00th January 20xx,
Accepted 00th January 20xx
Samuel Guerin*a and Brian E. Haydena,b
DOI: 10.1039/x0xx00000x
The development of functional perovskites for future technologies can be achieved though the combinatorial synthetic
method of evaporative Physical Vapour Deposition (HT-ePVD) which provides a direct low temperature route to anion
stoichiometric materials. When combined with the ability to control and vary precisely the composition of thin film libraries
of materials, high-troughput methods of screening and characterisation provides a rapid experimental determination of the
structure / function relationship. This review of the use of HT-ePVD shows that controlled cationic substitutions in A and/or
B sites can easily be explored, as can the effect of anionic substitution. This is exemplified in using this approach for a wide
range of perovskite systems, where the tuning of the functional properties through cation substitution has application in a
broad range of technologies.
www.rsc.org/
describe here how combinatorial synthesis by ePVD provides the first
step in a flexible and powerful high throughput methodology in
optimising the function of perovskites.
Introduction
Naturally occurring perovskites are formed in abundance within the
earth lower mantle, at depths between 660 and 2891 km with
temperatures and pressures of up to 4000 °C and 139 GPa at the
mantle – core boundary.1 Perovskites are an abundant natural
mineral, with 93 % of the earth’s lower mantle and 38 % of the earth,
by mass, being made up perovskites, MgSiO3 the commonest.2
However, these are limited to the high pressure of the earth’s lower
mantle and are not stable at ambient conditions. On the surface the
mineral CaTiO3 was the first perovskite discovered in 1839. However,
its interest and usage as a natural mineral has been limited. Following
characterisation, in 1926, of the crystal structure of calcium titanate3
and realisation of the ability to substitute, within the crystal
structure, a large range of cations and anions, perovskites have
received a large amount of attention due to their broad range of
applications. From dielectrics to electrical conductors, their
properties and usefulness become evident as their composition and
crystal symmetry changes. Non linear dielectrics become tunable
enabling their use in microwave applications. The creation of
localised electrical dipoles leads to polarizability, ferroelectricity and
piezoelectricity. Disorder and the creation of vacancies allow for
ionic transport. Perovskites can also be used as catalysts in fuel cells
for oxygen reduction and oxygen evolution reactions. Indeed,
perovskites materials find applications in photovoltaics,
superconductors, giant magneto-resistors, LEDs and lasers. We
Usually perovskites are made of alkaline earth or rare earth metals
in A sites and transition metals in B sites and the anions are mostly
oxygen, although many perovskites can also be formed using other
anions from the halogen group. The A sites occupy the corners of the
unit cell with a coordination number of 12 to the anions, whilst the B
sites occupy the centre of the unit cell with a coordination number
of 6. The anions form a network of octahedra and their ability to be
tilted is linked to many of the properties observed in dielectric
behaviours. The Goldschmidt tolerance factor using the relative size
of the ionic radii of atoms allow to predict the existence of perovskite
5
structures. Some articles4, summarise the possible use of the
elements in the three different sites of the perovskite structure in
the form of a periodic table. These are useful starting points when
embarking on high-throughput discovery of perovskite materials
with multiple elements.
Under ideal circumstances perovskites exhibit a pseudo-cubic
structure and distortions and variations in composition reduce the
crystal symmetry to form tetragonal, rhombohedral and monoclinic
crystal structures. Most studies focus on the study of balanced
perovskite where the A/B sites ratio equals one, however our
compositional spread approach allows to probe the compositional
spread further by easily achieving AxByO3-δ stoichiometries.
Being the subject of so many studies and such a large number of
applications, many routes are used for the synthesis of perovskites.
Bulk perovskite can be synthesised by classical approaches such as
solid phase reactions of common reagents, solution synthesis using
organometallic precursors. Other techniques such as hydrothermal
a. Ilika Technologies Ltd. Kenneth Dibben House, Enterprise Road, University of
Southampton Science Park, Chilworth, SO16 7NS.
b. Chemistry, The University of Southampton, Highfield, SO17 1BJ
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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processing, sol-gels or mechanochemical processes are also used to anions on the properties of the perovskites. It also allVoiewwsAtroticleenOsnulirnee
DOI: 10.1039/C9CC03927A
produce high quality perovskites. Similarly to how perovskites are stoichiometric concentrations of anions are provided during the
made in the natural world, high pressures are also used for their synthesis for all compositions investigated.
synthesis.6 If not already necessary during the synthesis steps, most
of these methods (apart from hydrothermal processing) will require
This approach has been used for the last ten years to probe the
elevated sintering conditions to finalise the synthesis step and
relationship between composition, structure and key figures of merit
of selected perovskites. As exemplified in this paper, perovskites
produce high density materials. Thin film perovskites have been
synthesised using CVD7-10, ALD11-15, PLD16-24, CSD25, 26sputtering27, 28
45, 46
used for piezoelectricity, such as PbxZryTi1-yO3-δ
and BixNd1-
and evaporative PVD29. However, most of these studies focus on
single compositions perovskites or a few discreet compositions. In
terms of using a high throughput approach, a number of studies refer
to computational methods making use of a limited number of
experimental points for comparison30-38. Approaches allowing the
rapid synthesis and screening of perovskite materials with extended
variation in the compositional space are limited by the synthesis
method used 21, 23, 24, 26-28, 39-44. Where some control of the cationic
concentrations can be achieved by combinatorial methods, the
necessity to employ high temperatures to ensure crystallisation can
lead to a difficulty in achieving stoichiometric anionic concentrations.
xFeyO3-δ47, ferroelectricity in BixNa1-xTiyO3-δ48, 49, tunable capacitors
50
BaxSr1-xTiy03-δ
,
BaxSr1-xTiyO3-zNz50 (Ba0.8-xGdxSr0.2 1+x
)
(Ti1-pZrp)1-xO3-
,
51
52
zNz.50 and BaxSr1-xTiyMn1-yO3-δ , lithium electrolytes as LaxLi1-xTiyO3-δ
54
and electrocatalysis with SrxTi1-yFeyO3-δ53, LaxCa1-xMnyO3-δ and
54
LaxMnyNi1-yO3-δ have all been studied using this approach. This
broad range of applications have all required different type of
substitution on either A-sites, B-sites, anionic sites or a combination
of all the above.
Perovskites systems studied by HT-ePVD
In our approach, we are able to fully control the compositional
spread of the cations enabling multiple substitutions in either
cationic sites, have some control of the anion site by being able to
offer a mixture of elements and have control of the temperature at
which synthesis takes place allowing control of the crystallinity of the
thin films. This is due to the co-evaporation of the constituent
elements in atomic form from the gas phases onto a moderately
heated substrate. As the atomic vapours for each cation form a
gradient of abundance across the substrate, each element is
condensing on the substrate in the desired quantity to generate the
target material. The relatively small distances atoms have to travel
to achieve equilibrium ensure that minimal energy is required to
form the target material, hence lower than typically used
temperatures have been necessary using this technique. The anions
are provided via an atom source which enables the dissociation of
the diatomic molecules into more reactive atomic species, these
have a high chemical potential allowing for the relatively easy
formation of stoichiometric perovskites. Furthermore, the synthesis
method will ensure that the anion concentration is balanced with the
cation concentration as the system minimised its energy during
synthesis. All these allow an unprecedented access to the
relationship existing between the properties under investigation and
their compositional dependence. High-throughput evaporative
Physical Vapour Deposition (HT-ePVD) offers the unique advantage
of allowing to control most aspect of perovskites synthesis. Although
ABO3 is the stoichiometric target composition, careful preparation of
the growth chambers allows to study the effect of elemental
substitutions on each of the cationic sites independently or
simultaneously. Furthermore, the use of an RF atom source to
provide the anionic element can be used as a source of mixed anions
thus enabling doping level investigations of the effect of mixed
Figure 1: sample photograph (A) and array map of the intensity of the (110) XRD
reflection (B) of
a LaxCa1-xMnyO3-δ library showing the link between the optical
observation of the perovskite and its existence as shown by XRD (system studied as part
of the work published in Bradley et al.54).
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Figure 1 shows the link between an actual photograph V(Aie)woAfrtaiclLeaOxCnlain1e-
xMnyO3-δ perovskite thin film sample and the map of intensity of the
DOI: 10.1039/C9CC03927A
(110) reflection obtained by XRD (B). In this particular sample the
domain of existence of the perovskite extends from 40 at% up to 56.6
at% of B-site atoms (Mn in this instance). This corresponds to a
Goldschmidt tolerance factor between 0.81 and 0.96 using standard
ionic radii, coordination numbers and averaged oxidation state of
Mn. This variation in the tolerance factor agrees with the reported
observation of both rhombohedral and cubic crystal structures. A
region with a particular optical reflectivity is observed corresponding
to the region of the perovskite phase. Outside of this region, the
sample remained amorphous, the colour variation between the two
regions appear as a shift / discontinuity and, although no thickness
measurement was obtained, is attributed a thickness difference
induced by a sudden change in density of the material as it changes
from amorphous to crystalline. The domain of existence of the
perovskite is restricted to a tolerance factor between 0.81 and 0.96
for this particular system and for the synthetic method used (the
samples were deposited by in-situ crystallisation through co-
deposition of constituent elements on a substrate at 550 °C) which is
relatively limited for our synthetic approach. Perovskite usually exist
with Goldschmidt tolerance factors (t) between 0.7 and 1.0 and their
crystal structure can be linked with its value.56 The ideal cubic
structure is reported to correspond to t between 0.9 and 1.0 whereas
distorted perovskites exits for t between 0.7 and 0.9. For other
systems synthesised and studied using our approach50, 54 we have
observed perovskites for the whole range of composition
Figure 2: Domain of existence of the LaxMnyNi1-yO3-δ perovskite structure as function of
composition. Existence expressed as the log of the sum of the area of the XRD peaks.
Calculated values of the Goldsmith tolerance factor overlay on the ternary showing
existence of the perovskite for 0.8 < t < 1.2 (system studied as part of the work
published in Bradley et al.54).
synthesised with
a corresponding Goldsmith tolerance factor
between 0.82 and 0.98. More remarkable is the fact that using our
synthesis method we are able to obtain perovskite structures for t>1,
when at these values non-perovskites structures are normally
reported56. For the BaxSr1-xTiyO3-zNz pure perovskite was reported for
t up to 1.05 and mixed phase above that. A more remarkable domain
of existence is highlighted in the ternary plot of Figure 2 where the
log of sum of the area of the diffraction peaks of the perovskite is
plotted against composition and overlaid with the calculated
Synthesis
All systems presented have been synthesised using
a high-
throughput evaporative physical vapour deposition system using a
system of independent wedge shutters and sources allowing the co-
deposition of the constituent elements.55 All elements were
evaporated from their pure solid forms by generating a vapour tolerance factor. From this figure we can infer that, for the LaxMnyNi1-
yO3-δ system, a perovskite can be formed for tolerance factor
between 0.8 and 1.2.
allowing the control of their relative deposition rates. The cations
were usually contained in crucibles and loaded using either Knudsen
cells or electron gun sources depending on the temperature required
to achieve a sufficient deposition rate. Anions were provided by
feeding a flux of pure (or mixed) molecular atoms to an RF atom
source where a plasma contained within a discharge tube allow for
the splitting of the di-atomic molecules into a mixture of molecules,
ions, charged radicals and neutral atoms. This mixture being much
As mentioned before, the formation of the perovskites was usually
done by either, deposition at moderate temperature followed by
annealing to ensure crystallisation or by in-situ deposition to allow
direct formation of the perovskite structure as the elements are
evaporated and the film grown. Generally, the range of compositions
more reactive than the molecular gas allow for the synthesis of anion for which the perovskites is observed is greater for the direct in-situ
crystal growth method than in the case of the post deposition
annealing approach. Furthermore, trends attributable to the
variations in the relative ratios of A-sites and B-sites cations become
more apparent and extend more easily to super and sub
stoichiometry of the anions in the case of in-situ crystal growth
compared to post deposition annealing. Therefore, when
experimentally possible, it is preferred to synthesise the perovskites
by in-situ direct crystallisation.
stoichiometric compounds: The anions were provided in excess of
the cations, at all positions of the substrate, in order to favour the
stoichiometric compounds to be formed for a given cationic
concentration. The syntheses presented here were carried out
mainly on platinised substrates at
a moderate temperature,
(followed by an annealing step to ensure formation of the perovskite
structure) or by direct synthesis at an elevated substrate
temperature to favour the in-situ growth of the crystalline phase as
the elements are being deposited.
A-site substitutions
A-site substitution has been the commonest approach with the
studies of BixNa1-xTiyO3-δ as ferroelectrics48, 49, BixNd1-xFeyO3-δ as
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piezoelectric47, BaxSr1-xTiyO3-δ as tunable dielectrics50, LaxLi1-xTiyO3-δ The LaxLi1-xTiyO3-δ system was studied for its properties as Li ion
View Article Online
as lithium ion conductor52 and LaxCa1-xMnyO3-δ as electrocatalysts.54
52
DOI: 10.1039/C9CC03927A
conductor , its synthesis was achieved by post deposition annealing
of the thin films and multiple samples were required to allow for a
computational analysis of the experimental data obtained which
Synthesis of the BixNa1-xTiyO3-δ, BixNd1-xFeyO3-δ and LaxLi1-xTiyO3-δ
yielded a hotspot for ionic conductivity (5.45 x 10-4 S cm-1 for
systems was done by depositing the constituent elements at
Li0.17La0.29Ti0.54 (excluding oxygen)). The mechanism for Li ion mobility
moderate substrate temperature resulting in amorphous thin films
within the structure is due to the large number of vacancies within
which required a post deposition annealing step to form the
the perovskite structure as this is reflected in its more common
perovskite structure. For the BaxSr1-xTiy03-δ and LaxCa1-xMnyO3-δ the
formula of Li3xLa(2/3)-x□
(1/3)-2xTiO3 (0 < x < 0.16).57
perovskite was formed by direct in-situ crystal growth achieved by
deposition of the elements at elevated substrate temperatures (650
°C for BaxSr1-xTiyO3-δ and 550 °C for LaxCa1-xMnyO3-δ).
The dielectric properties of BaxSr1-xTiy03-δ were investigated for its
application in antenna as tunable dielectric.50 This application
requiring thin film as final product ensure that the study’s findings
have direct bearing on product development. This system was
In the case of the BixNa1-xTiyO3-δ system48 a map of the dielectric
constant variation (see Figure 3) shows a gradual increase from Ti
rich compositions toward a maximum for the ideal cation ratio at 50
at% Ti, past this line there is a sudden decrease in the measured
dielectric constant. A region of relatively high dielectric constant is
observed for around 30 at % Bi, 20 at% Na and 50 at% Ti (excluding
oxygen) and it is surrounded by regions where little to no polarisation
is measured.
synthesised by direct in-situ crystallisation, with
a substrate
temperature of 650 °C, leading to entire sample libraries with wide
compositional spread exhibiting pure perovskite phases. The
resulting dielectric map obtained is shown in Figure 4, it shows a
region of higher dielectric constants for Ba rich compositions,
compared to Sr, and these are found for slight excess Ti (with respect
to the target 50 at% perovskite). For this system a tunability ratio of
4.9 was achieved between 0 and 36 kV mm-1.
Figure 3: Dielectric constant for the BixNa1-xTiyO3-δ system following anneal in air at 700
C.48
Figure 4: Dielectric constant for the BaxSr1-xTiy03-δ system (results obtained as part of the
work published by David et al.50).
Piezoelectricity at room temperature was the focus of the study in
the BixNd1-xFeyO3-δ system47 and its direct measurement by PFM was
limited to the tie line of the ideal perovskite BixNd1-xFeO3, a phase
transition was identified between a R3c and Pnma crystal structure
with a region of mixed phases for 0.12 < x < 0.2, and an increase in
piezoelectric response was observed with a maximum at around x =
0.11. It was concluded that the maximum was linked to the boundary
between the single R3c phase and the mixed phase region caused by
a softening of the lattice in the vicinity of the phase boundary.
B-site substitution
B-site substitution studies have focussed on studying the PbxZryTi1-
yO3-δ solid solution for piezoelectric materials45, 46 and LaxMnyNi1-yO3-δ
and SrxTi1-yFeyO3-δ as electrocatalysts.53, 54
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The application of high-throughput thin film methodologies for the (ORR).53, 54 In the case of the SrxTi1-yFeyO3-δ53, it was VfoieuwnAdrtictlhe aOtnlainne
extensive domain of existence of the peroDvOskI:it1e0.1w0a3s9/oCb9tCaCin0e3d92b7Ay
synthesis at 650 °C. Interestingly not only the range of compositions
in the solid solution of SrxTi1-yFeyO3-δ achieved was larger using the
ePVD method compared to traditional methods but also the
temperature at which this was achieved is significantly lower. The
extensive characterisation and testing undertaken in this system
concluded that the OER was found to be optimal for the SrTi0.5Fe0.5O3-
synthesis and study of perovskite materials was first developed using
our approach for the study of piezoelectric materials such as lead
zirconate – lead titanate with the exploration of the PbxZryTi1-yO3-δ
46
solid solution.45, These libraries were made by post deposition
annealing to obtain the perovskite structure. Analysis of the crystal
structure via deviation from Vegard’s law (see Figure 5 top)
combined with Raman spectroscopy revealed a transition at 36 at%
Ti (with respect to Zr) between a tetragonal phase (Ti rich) to a
rhombohedral one (Zr rich). Thin film XRD proved unable to resolve
the difference between the two crystal structures, both appearing
pseudo cubic. The presence of this morphological phase boundary is
similar to what is observed in bulk PZT although it is reported for bulk
to happen for a Zr:Ti ratio of 0.53:0.47. The difference has been
attributed to residual stress and smaller grain sizes.46 For selected
compositions along the solid solution the ferroelectric response was
obtained with a clear trend of large polarisation for Zr rich
compositions as shown in Figure 5 (bottom).
composition as it offers the best compromise between catalytic
δ
activity and electrode stability. Furthermore, mechanisms involving
oxygen vacancies are believed to be responsible for the relatively
high activity of the materials. A similar synthetic approach was taken
for the study of LaxMnyNi1-yO3-δ system.54 A relatively low substrate
temperature of 550 °C was sufficient to ensure crystallisation of the
perovskite structures along the lanthanum tie-line. The LaxMnyNi1-
yO3-δ system is found vary from cubic for LaMnO3 to rhombohedral
for LaNiO3 with evidence of an unexpected A-site substitution of
Mn2+ in LaMnO3 and Ni2+ in LaxMnyNi1-yO3-δ. Electrochemical activity
shows an expected anti-correlation between OER and ORR.
However, in the case of the observed A-site substitution of Ni2+, a
correlation OER and ORR is observed, linked to the appearance of a
Mn3+/Mn4+ redox couple in the electrochemistry. The optimal
reversible ORR/OER activity is observed for La0.85Mn0.7Ni0.45O3-δ, a
composition away from the tie line for a simple B-site substitution.
This study was completed by the synthesis and study, in a similar
manner, of the expected A-site substitution of LaxCa1-xMnyO3-δ. This
system was observed to vary from cubic to cubic for the end
members via an orthorhombic crystal structure as La substitutes for
Ca along the Mn tie-line. The results show that the substitution of
La3+ for Ca2+ results in the appearance mixed valent perovskites with
an optimum OER/ORR activity linked to the appearance of the
Mn3+/Mn4+ redox couple.
A and B site substitutions
The latest example shows the link between A and B site substitutions;
the HT-ePVD synthetic approach allows also studies of both A and B
sites substitutions simultaneously. This has been the focus of further
optimisation of the BST tunable dielectric where the BaxSr1-xTiyMn1-
yO3-δ system was studied for its use in tunable co-planar waveguides
for GHz applications.51 Libraries of BaxSr1-xTiyMn1-yO3-δ were
synthesised by in-situ growth with a substrate temperature of 640 °C
to ensure formation of the perovskite. The region of interest
focussed on small compositional variations around the Ba0.8Sr0.2TiO3
target with Mn doping of up to 5 at%. The dielectric properties were
tested under varying dc bias fields at three different frequencies (1,
10, 100 kHz) and tunabilities of 58.3 % and 57.3 % at 1 and 100 kHz,
respectively, have been obtained between 0 and 45 kV mm-1 bias.
Although the tunability has decreased by 7-8 % with respect to the
undoped BST, one of the major advantages gained by Mn doping was
to lower the dielectric loss and to allow stable dielectric
performances at elevated frequencies. The material was also used in
a co-planar waveguide phase shifter and was shown to operate at
over 10 GHz with a low insertion loss of ~3.2 dB, a 12° phase shift was
Figure 5: variation in unit cell volume (top) for the PbZryTi1-yO3 system. Ferroelectric loops
for selected regions along the solid solution of PbZryTi1-yO3.46
achieved with
a
small 10
V
biasing voltage with excellent
transmission characteristics.
Direct in-situ synthesis of B-site substitutions have been the focus of
two studies with the purpose of optimising electrocatalysts used for
Oxygen Evolution Reaction (OER) and Oxygen Reduction Reaction
Anions doping and multiple doping
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So far, all approaches using HT-ePVD have focussed on cationic sites
and their substitutions. Although most perovskites are oxygen
perovskites, to the extent that the generic formula is often referred
to as ABO3, there are other elements in the periodic table which can
go on the anionic site of the perovskite structure, such as fluorine,
chlorine, bromine, iodine, sulphur and selenium. In fact, a favoured
generic formula for perovskites is ABX3.5 Another possibility for
anionic site is to mix or substitute the anions. Recently an organic-
inorganic solar cell perovskite with a sulphur and iodine mixture has
been reported.58 A similar concept was used in 2013 when we have
used the HT-ePVD approach to the synthesis and study of anion
doping combined with A site substitutions in BaxSr1-xTiy(O,N)3-δ as
tunable dielectrics. This was also extended into a study of
substitutions on all sites with the investigation into the (Ba0.8-
View Article Online
DOI: 10.1039/C9CC03927A
xGdxSr0.2)1+x ,
(Ti1-pZrp)1-xO3-zNz.50 In the case of the BaxSr1-xTiy(O,N)3-δ
pure perovskite phases were obtained by in-situ crystallisation for a
large range of compositions. Secondary phases are detected for
compositions away from the tie-line with a tolerance factor in excess
of 1.05. Significant improvements in the electrical characteristics of
the materials were also obtained with an increase in dielectric
constant when compared to the pure oxide perovskite and a
tunability of 6.8 between 0 and 36 kV mm-1 at 1 kHz. Decreases in the
dielectric losses were also seen under biases compared to the pure
oxide perovskite with values of tan δ as low as 0.0032 at 36 kV mm-1
and 1 kHz. The ternary plot of dielectric constant shown in Figure 6
reveals a region where values between 900 and 1000 have been
measured, this region corresponds to a higher concentration of
barium than of strontium (similarly to what is obtain for the BaxSr1-
xTiyO3-δ system) however it also corresponds to titanium
concentrations slightly lower than the ideal 50 at% tie-line. This is the
opposite to what is observed in the case of the BaxSr1-xTiyO3-δ system
(see Figure 4). The difference has been observed repeatedly and it is
not clear if the nitrogen is playing a direct role in this observation or
if its influence is limited to the crystal growth, the oxynitride thin
films were consistently found to have significant preferential
orientation with a very strong (001) reflection. The levels of nitrogen
measured by XPS in these samples is low, nonetheless the
improvements in electrical performance are significant. Another
aspect to notice is that the apparent hot spot of highest dielectric
constant seem parallel to the Ti concentration for BaxSr1-xTiyO3-δ but
deviates towards lower Ti levels in the BaxSr1-xTiy(O,N)3-δ. Possible
explanations involve oxygen vacancies, change in oxidation levels or
site substitutions, further experiments would be required to further
elucidate this. Finally the study of gadolinium and zirconium doping
of the BaxSr1-xTiy(O,N)3-δ system involved multiple substitutions
between Gd, Ba and Sr on the A-site, Zr and Ti on the B-site and
nitrogen and oxygen on the anionic site. Using the in-situ direct
crystallisation approach, the samples were deposited with a
substrate temperature of 650 °C. A single perovskite phase was
obtained for the compositional spread studied and although no
improvements in dielectric constant, tunability and dielectric loss
were observed, compared to the maxima found in the BaxSr1-
xTiy(O,N)3-δ, it was found that regions of low dielectric loss extended
for a wider range of compositions.
Figure 6: Dielectric constant at 1 kHz for the BaxSr1-xTiy(O,N)3-δ system (results obtained
as part of the work published by David et al.50).
Experimental Procedures for HT-ePVD studies
The syntheses took place a UHV system with a base pressure of 2x10-
9 Torr by DCA Instruments. The elements (from 97% to 99.9% purity)
were deposited from either Knudsen cells (25 cc or 40 cc) from DCA
instruments or electron beam sources (40 cc) from Telemark, the
gases were provided via a RF atom source from Oxford Applied
Research with a 600 W RF power. The substrates used were either
SSTOP (Si/SiO2/TiO2/Pt), AlOPt (sapphire/TiO2/Pt) or ITO
electrochemical substrates.
All characterisations and screening were done, typically, as a 14x14
array of measurements, although arrays of 10x10, 12x12 and 8x8
have also been used.
Compositional characterisation was done using either an SEM
equipped with an EDS system or Laser Ablation Inductively Coupled
Plasma Mass Spectrometry (LA-ICPMS). X-ray diffraction was done
using a Brucker Discovery D8 equipped with an Incoatec microsource
Cu Kα and GADDS detector. XPS studies were done using a Spot XPS
(ThermoFisher XR3 twin anode with Alpha 110 analyser and
Advantage software). Electrical characterisations were done using an
LCR meter (HP4284A), an Impedance Analyser (Solartron 1260A) and
I-V source meters (Keithley) used in conjunction with a Signatone
probestation. Electrochemical characterisation was carried out with
samples specially deposited on 100 electrodes electrochemical
arrays and tested using custom built cells, current followers and
potentiostat.
Further information, specific to each system, about the experimental
details can be found in the relevant published work. 45-50, 52-55
Conclusions
6 | J. Name., 2012, 00, 1-3
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ARTICLE
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The use of High-Throughput evaporative Physical Vapour
Deposition has been extensively used for the synthesis and
study of perovskite thin films. The unique ability to control large
compositional variations allows to gain an unrivalled insight into
the material properties, by providing a large set of experimental
points across the compositional space, compared to more
classical approaches where only set of discreet compositions
can be studied. The co-deposition, by condensation, of the
constituent elements from vapour phases in vacuum, lead to a
low energy route to phase formation which allow pure anion
stoichiometric crystalline phases to be obtained at lower
temperatures than in the case of more traditional approaches.
The direct in-situ crystallisation route by deposition on
substrates at elevated temperature is favoured but not always
practical due to the low vapour pressures of some elements
resulting in low residence times on the substrate. Our approach
applied itself to substitution studies on both cationic sites as
well as the anionic site and interpretation of the data obtained
with respect to compositional space can also highlight
unexpected sites substitutions. This approach is also unique in
allowing to generate large datasets for many applications of
interest, these relatively easily accessible experimental
datapoints are essential in building databases for enabling the
development of reliable algorithms to use with the developing
field of machine learning. Finally, this approach has been
extensively used for the synthesis and study of the perovskite
class of materials, but it does not limit itself to these. Any crystal
class, such as, for example, pyrochlores, spinels, garnets and
delafossites, where systematic and gradual elemental
substitutions are possible, presents an opportunity to deploy
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors would like to acknowledge Dr Chris Vian for
provision of support in informatics.
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