Generalized synthesis of NaLn(MoO4)2 nano/microcrystals (Ln = La–Lu and Y): The effects of lanthanide contraction, structure, and down-/up-conversion luminescence

Article abstract of DOI:10.1016/j.jallcom.2020.154676

Systematic synthesis of scheelite-type double molybdate nano-/microcrystals NaLn (MoO₄)₂ (Ln = La–Lu lanthanides and Y) was successfully carried out via hydrothermal reaction at 180 °C without using any organic additive, and the intrinsic effects of lanthanide contraction on phase preference and crystallite morphology were unambiguously observed. It was shown that the products of larger Ln³⁺ (Ln = La–Dy) and smaller Ln³⁺ (Ln = Ho–Lu and Y) elements crystallized as tetragonal NaLn (MoO₄)₂ and new orthorhombic NaLnMo₂O₈·2H₂O structures, respectively, with the latter being able to dehydrate to tetragonal NaLn (MoO₄)₂ upon calcination at ～300 °C. Y was found to be a demarcation point for the two structures, and phase preference was shown to be influenced by solution pH and MoO₄^2?/Y³⁺ molar ratio. With Eu³⁺ as a downconversion (DC) luminescence probe, the effects of Ln type (Ln = La, Gd, Y and Lu) and crystal structure were manifested. Furthermore, the upconversion (UC) luminescence of Yb³⁺/Ho³⁺ and Yb³⁺/Er³⁺ pairs in NaLu(MoO₄)₂ was studied for the first time, and strong red and green UC emissions were observed under the 978 nm laser excitation. Besides, the processes/mechanisms of UC were studied via varying the excitation power and were discussed with Yb³⁺-MoO₄^2- dimer sensitizer.

Full text of DOI:10.1016/j.jallcom.2020.154676

Journal of Alloys and Compounds 830 (2020) 154676
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Generalized synthesis of NaLn(MoO4)2 nano/microcrystals (Ln ¼
LaeLu and Y): The effects of lanthanide contraction, structure, and
down-/up-conversion luminescence
,
b
,
,
,
, b, c
Zhixin Xu a c, Siyuan Li a b, Qinghong Meng a b, Xuejiao Wang d, Qi Zhu a
,
Ji-Guang Li c
,
*
a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang, Liaoning, 110819, PR China
b Institute of Ceramics and Powder Metallurgy, School of Materials Science and Engineering, Northeastern University, Shenyang, Liaoning, 110819, PR China
c Research Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan
d College of New Energy, Bohai University, Jinzhou, Liaoning, 121000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Systematic synthesis of scheelite-type double molybdate nano-/microcrystals NaLn (MoO4)2 (Ln ¼ LaeLu
lanthanides and Y) was successfully carried out via hydrothermal reaction at 180 ꢀC without using any
organic additive, and the intrinsic effects of lanthanide contraction on phase preference and crystallite
morphology were unambiguously observed. It was shown that the products of larger Ln3þ (Ln ¼ LaeDy)
and smaller Ln3þ (Ln ¼ HoeLu and Y) elements crystallized as tetragonal NaLn (MoO4)2 and new
orthorhombic NaLnMo2O8$2H2O structures, respectively, with the latter being able to dehydrate to
tetragonal NaLn (MoO4)2 upon calcination at ~300 ꢀC. Y was found to be a demarcation point for the two
structures, and phase preference was shown to be influenced by solution pH and MoO24ꢁ/Y3þ molar ratio.
With Eu3þ as a downconversion (DC) luminescence probe, the effects of Ln type (Ln ¼ La, Gd, Y and Lu)
and crystal structure were manifested. Furthermore, the upconversion (UC) luminescence of Yb3þ/Ho3þ
and Yb3þ/Er3þ pairs in NaLu(MoO4)2 was studied for the first time, and strong red and green UC emis-
sions were observed under the 978 nm laser excitation. Besides, the processes/mechanisms of UC were
studied via varying the excitation power and were discussed with Yb3þ-MoO42- dimer sensitizer.
© 2020 Elsevier B.V. All rights reserved.
Received 2 December 2019
Received in revised form
16 February 2020
Accepted 5 March 2020
Available online 6 March 2020
Keywords:
NaLn(MoO4)2
Hydrothermal synthesis
Lanthanide contraction
Photoluminescence
1. Introduction
which many efforts were devoted to controllable synthesis of ALn
(MoO4)2 nano/microstructures, particularly through hydrothermal/
Scheelite-type ALn (MoO4)2 double molybdates (A ¼ alkali;
Ln ¼ lanthanide or Y) received much research interest for opto-
electronic and luminescent applications due to their good chemi-
cal/physical stability, facile substitution of the Ln3þ site with rare-
earth (RE3þ) activators and the sensitization effect of [MoO4]2-
ligand [1,2]. In these compounds, the Aþ and Ln3þ ions occupy the
same lattice position, resulting in a highly disordered tetragonal
crystal structure that allows wide absorption of the excitation light
and high doping concentration of RE3þ elements [3e5].
solvothermal reactions. The successful examples may include
monodisperse NaLa(MoO4)2 bipyramid nanocrystals formed using
oleic acid/oleylamine co-surfactant [6], urchin-like NaY(MoO4)2
microarchitectures prepared with citrate (Cit3ꢁ) chelate [7], rugby-
shaped NaEu(MoO4)2 particles and NaLn (MoO4)2 microsquares
(Ln ¼ Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er) prepared with EDTA
chelate [8], and rhombic-shaped NaLn (MoO4)2 submicron crystals
(Ln ¼ Yb, Tm, Er, Y, Dy, Gd, Eu, Sm and Ce) fabricated using water/
ethanol co-solvent and oleic acid surfactant [9]. In the absence of
any template, surfactant or organic additive, the effects of hydro-
thermal parameters on characteristics of the hydrothermal product
have been examined by a number of researchers. Li et al., for
example, systematically studied the influence of MoO24ꢁ/Gd3þ
molar ratio and solution pH on phase structure and crystallite
morphology, and the NaGd(MoO4)2 microcrystals in the forms of
bipyramids, truncated bipyramids, quasi-cubes and tetragonal
The physicochemical property of a powder material is known to
be heavily dependent on its particle geometric size and shape, for
* Corresponding author. National Institute for Materials Science, Tsukuba, Ibaraki,
Japan.
E-mail address: LI.Jiguang@nims.go.jp (J.-G. Li).
https://doi.org/10.1016/j.jallcom.2020.154676
0925-8388/© 2020 Elsevier B.V. All rights reserved.

2
Z. Xu et al. / Journal of Alloys and Compounds 830 (2020) 154676
plates were selectively synthesized [4]. The well-dispersed NaLa(-
MoO4)2 hierarchical microcrystals with diverse morphologies (such
as microspheres, ellipsoids, microspindles and microflowers) have
also been obtained by manipulating solution pH, hydrothermal
temperature and solvent composition [10].
temperature, the precipitate was centrifuged and washed with
deionized water and ethanol four times before drying in air at 50 ꢀC
for 24 h. The calcination of hydrothermal product was performed in
flowing oxygen (200 mL/min), using a heating rate of 10 ꢀC/min and
a holding time of 2 h at each temperature (up to 500 ꢀC). 5 at% Eu3þ
(relative to Ln3þ) doped NaLn (MoO4)2 downconversion (DC)
phosphors (Ln ¼ La, Gd, Y, and Lu) and 10 at% Yb3þ (relative to Lu)
and 2 at% RE3þ (RE ¼ Ho, Er; relative to Lu) codoped NaLu(MoO4)2
upconversion (UC) phosphors were synthesized following the
above procedures.
Despite the aforementioned successes, our survey through the
literature indicated that general synthesis of NaLn (MoO4)2 nano/
microcrystals for the full spectrum of lanthanides and Y (a transi-
tion metal with ionic radius between those of Dy and Ho) was not
achieved via the hydrothermal technique, even with various sur-
factants and chelates for composition, structure and morphology
control. The lanthanide elements exhibit periodically changing
electronic configuration and, therefore, physical and chemical
properties with increasing atomic number, which is known as
lanthanide contraction. In this regard, a systematic investigation
yet lacks for the effect of lanthanide contraction on the hydro-
thermal crystallization, structure and morphology of NaLn (MoO4)2,
though this has been shown for other types of lanthanide com-
pounds, such as hydroxides, phosphates, orthovanadates and
fluorides [11e16]. In this work, we developed an organic-free hy-
drothermal strategy for generalized synthesis of NaLn (MoO4)2
nano-/microstructures (Ln ¼ LaeLu lanthanides and Y), and the
intrinsic influence of lanthanide contraction was unveiled. It was
clearly shown that tetragonal structured NaLn (MoO4)2 and
orthorhombic structured NaLnMo2O8$2H2O can be directly crys-
tallized for the larger Ln3þ ions of Ln ¼ LaeDy and the smaller Ln3þ
of Ln ¼ HoeLu and Y, respectively, and the latter can be facilely
transformed to tetragonal NaLn (MoO4)2 via dehydration at the
very low temperature of ~300 ꢀC. It was also found that Y3þ
happened to be the demarcation point for the two above
mentioned structure types, and the effects of solution pH and
MoO24ꢁ/Y3þ molar ratio on phase preference were revealed. Finally,
the downconversion (DC) photoluminescence of Eu3þ, which is
sensitive to local coordination and site symmetry [17e19], was
investigated for the four typical samples of Ln ¼ La, Gd, Y and Lu to
show the effects of Ln type and crystal structure. Furthermore,
2.2. Characterization
The phase identification was performed by X-ray diffractometry
(XRD, Model SmartLab, Rigaku, Tokyo, Japan) under 40 kV/40 mA,
using nickel-filtered Cu K
/min. The XRD data for Rietveld refinement of crystal structure
a
radiation and a scanning speed of 5.0ꢀ
2q
were acquired in the step-scan mode, using a step size of 0.02ꢀ and
an accumulation time of 1.9 s per step. The product morphology
and structure were analyzed via field emission scanning electron
microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo) under 15 kV
and transmission electron microscopy (TEM, Model JEM-2000FX,
JEOL, Tokyo) under 200 kV. Fourier transform infrared spectros-
copy (FTIR, Nicolet iS5, Thermal Fisher Scientific, USA) was con-
ducted by the standard KBr pellet method. Thermogravimetry (TG,
Model SETSYS Evolution-16, Setaram, France) was carried out in
flowing oxygen (100 mL/min) under a constant heating rate of
10 ꢀC/min. DC and UC luminescence were analyzed with a Model
FP-8600 fluorospectrophotometer (Jasco, Tokyo) under a scan
speed of 200 nm/min, using a 150 W xenon lamp and a 978 nm
emitting continuous wavelength (CW) laser diode (Model KS3-
12322-105, BWT Beijing Ltd., Beijing, China) as excitation sources,
respectively.
3. Results and discussion
considering that NaLu(MoO4)2 can be an ideal host lattice for Yb3þ
/
3.1. Crystal structure and morphology
Ho3þ and Yb3þ/Er3þ doping, owing to minimal lattice distortion by
the very similar ionic sizes of these dopants and host Lu3þ ions, the
upconversion (UC) luminescence of NaLu(MoO4)2:Yb3þ,RE3þ
(RE ¼ Ho, Er) phosphors, which has hardly been reported in the
literature, was also clarified in this work. In the following sections,
we report the synthesis, characterization, and luminescence
properties of the series of double molybdates NaLn (MoO4)2.
XRD analysis found two categories of products for the series of
lanthanide and Y elements, though the conditions of hydrothermal
reaction are the same (at 180 ꢀC for 24 h, MoO24ꢁ/Ln3þ ¼ 5, pH ¼ 6).
The first group (Fig. 1A), for the larger Ln3þ of Ln ¼ LaeDy in the
lanthanide family (9 elements, excluding radioactive Pm), directly
crystallized as the targeted NaLn (MoO4)2 double molybdates and,
in each case, all the diffraction peaks can be readily indexed with
those of the known tetragonal structured NaLa(MoO4)2 (space
group: I41/a; JCPDS 24e1103) [20]. The Rietveld refinement of XRD
pattern recorded for the NaLa(MoO4)2 representative (Fig. S1A) was
ended up with satisfactory R-factors and it showed that the
experimental and calculated XRD patterns well coincide with each
other, which further confirmed the pure phase product formation.
The refined cell parameters of a ¼ b ¼ ~5.348 (5) Å and c ¼ ~11.75
(1) Å and cell volume V ¼ ~336.1(8) Å3 (Table S1) are close to those
of tetragonal NaLa(MoO4)2 (a ¼ b ¼ 5.3430(1) Å, c ¼ 11.7437(3) Å,
and V ¼ 335.24 Å3, JCPDS 24e1103) [20]. From the schematic
illustration of the tetragonal unit cell (Fig. S1B), it is seen that Mo6þ
is coordinated with four oxygen atoms to form MoO4 tetrahedron,
while Naþ and La3þ are coordinated with eight oxygen atoms from
four adjacent molybdate ligands and randomly occupy the do-
decahedron center [21].
2. Experimental
2.1. Reagents and sample synthesis
99.99% pure lanthanide sesquioxides (Ln2O3, RE ¼ La, Nd, Sm,
Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Y), Pr6O11, Tb4O7 and Ce(NO3)3$6H2O
were purchased from Huizhou Ruier RareChem. Hi-Tech. Co. Ltd
(Huizhou, China). Analytical grade Na2(MoO4)$2H2O, NaOH, and
HNO3 were purchased from Shenyang Chemical Reagent Factory
(Shenyang, China). The nitrate solution of Ln3þ (except for Ce3þ
)
was obtained by dissolving the corresponding oxide in an appro-
priate amount of HNO3 solution. In a typical synthesis, 4 mL of Ln3þ
solution (0.5 mol/L) was added into 65 mL of Na2(MoO4)$2H2O
solution (Mo/Ln ¼ 5:1 molar ratio) under magnetic stirring at room
temperature, followed by pH adjustment to a desired value
(pH ¼ 4e8) using dilute HNO3 and NaOH solutions. The mixture,
after homogenization by constant magnetic stirring for 30 min, was
transferred into a Teflon-lined stainless steel autoclave of 100 mL
capacity, followed by 24 h of hydrothermal reaction in an electric
oven preheated at 180 ꢀC. After natural cooling to room
The second category of hydrothermal products, identified for
the smaller Ln3þ of Ln ¼ HoeLu and Y (Fig. 1B; 6 elements), showed
XRD patterns that can not match with any known compound in the
JCPDS database but conform to the orthorhombic structured
NaLuW2O8$2H2O compound (Cmmm space group) recently

Z. Xu et al. / Journal of Alloys and Compounds 830 (2020) 154676
3
Fig. 1. XRD patterns for the hydrothermal products of Ln ¼ LaeDy (A) and Ln ¼ HoeLu and Y (B).
reported by us [22,23]. With the Lu-containing sample as repre-
sentative, the elemental mapping (Fig. S2) showed that the Na, Lu,
Mo and O elements are uniformly distributed over the crystallites,
which suggests a single-phase nature of the product. The Rietveld
refinement of the XRD pattern (Fig. 2) was ended up with the well
acceptable reliability factors of Rwp ¼ ~7.74%, Rp ¼ ~5.30%,
Rexp ¼ ~5.91%, RB ¼ ~1.21% and c2 ¼ ~1.31 (Table S1), which further
confirmed phase purity and crystal structure of the Lu product. The
cell parameters (a ¼ ~21.767(5) Å, b ¼ ~5.033(1) Å, c ¼ ~3.802(6) Å)
and cell volume (V ¼ ~416.6(1) Å3) derived for NaLuMo2O8$2H2O
(Table S1) are larger than those of NaLuW2O8$2H2O except for the b
value (a ¼ 21.655 Å, b ¼ 5.1352 Å, c ¼ 3.6320 Å, and V ¼ 403.87 Å3)
[23], which suggests that the structure of NaLuMo2O8$2H2O sub-
stantially shrink in the b direction. The stimulated crystal structure
(Fig. 2, the inset) shows that Naþ ions are coordinated with six O2ꢁ
ions to form octahedron, which is connected to each other by
sharing faces and edges. The Lu3þ ion is coordinated by four O
atoms from MoO5 trigonal bipyramids and two O atoms from H2O
molecules to form LuO4(H2O)2 octahedron, where the H2O mole-
cules are terminal and isolated but can be connected to other O
atoms via hydrogen bonds. Each pair of neighboring MoO5 are
connected by sharing one edge and connected to the other two
MoO5 through common vertices. In addition, the NaO6 and
LuO4(H2O)2 polyhedrons are joined together through sharing O
atoms with MoO5.
The above results clearly indicated that tetragonal NaLn(MoO4)2
and orthorhombic NaLuW2O8$2H2O are preferred by Ln3þ ions
larger than Y3þ (1.019 Å) and smaller than Y3þ (1.019 Å), respec-
tively. Our experiments also identified that Y3þ is the demarcation
point for the two structures, possibly because the size of Y3þ is
located between those of Dy3þ and Ho3þ [24]. At the same time, it
was noticed that Ln3þ has the different coordination numbers of 8
and 6 in the tetragonal and orthorhombic structures, respectively.
The structure and coordination transformation could be due to a
spatial effect caused by lanthanide contraction, which weakens the
Ln-O bond with decreasing Ln3þ size [16]. As a result, a small
enough Ln3þ cannot coordinate with as many oxygen atoms as in
the NaLn(MoO4)2 structure.
In Fig. 3, the FTIR spectra are presented for the full series of
hydrothermal products, which would help to understand chemical
composition and local coordination. Previous reports [25e27]
indicated that only the F2 (y3, y4) vibration modes are IR active for a
MoeO polyhedron in tetrahedral symmetry. Therefore, for the
Ln ¼ LaeDy series (NaLn(MoO4)2, Fig. 3A), the strong and broad
absorption ranging from ~903 to 717 cmꢁ1 and the weak one at
~404 cmꢁ1 can be attributed to the F2 (
and F2 (y
y
3) antisymmetric stretching
4) antisymmetric bending vibrations of MoeO bonds,
respectively. The broad range of F2 3) is owing to vibration
(
y
coupling, since the four MoeO bonds, the same in bond length (the
inset in Fig. 3A), have similar vibration frequencies. Besides, the
negligibly weak absorptions centered at ~3450 and 1630 cmꢁ1 are
arising from the OeH stretching (y1 and y3) and HeOeH bending
(y2) modes of adsorbed water molecules [28,29]. The Ln ¼ HoeLu
and Y series (NaLnMo2O8$2H2O) exhibited better split vibrations in
a wider spectral region (Fig. 3B), since the compounds have three
MoeO bond lengths (Table S2), which lowered the symmetry of
MoeO polyhedron. Considering that a shorter bond would make
MoeO vibration more difficult to occur, the ~932, 860 cmꢁ1 and
770/695 cmꢁ1 bands, decreasing in vibration energy, can be
assigned to MoeO4, MoeO2 (two equivalent bonds) and MoeO3
(two equivalent bonds) vibrations, respectively (the inset in
Fig. 3B). The double splitting of MoeO3 vibration arises from the
influence of Ln atoms, since O3 is connected to both Mo and Ln.
Additionally, the ~1633 and 3485 cmꢁ1 vibrations of H2O are
Fig. 2. Rietveld difference plot for NaLuMo2O8$2H2O, where the red open circles, black
line, grey line and green tick marks are for experimental pattern (Yexp), calculated
pattern (Ycalc), the difference between experimental and calculation (Yexp-Ycalc), and
Bragg positions of diffraction, respectively. The insets schematically present the cell
unit and local coordinations. (For interpretation of the references to color in this figure
legend, the reader is referred to the Web version of this article.)

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Z. Xu et al. / Journal of Alloys and Compounds 830 (2020) 154676
Fig. 3. FTIR spectra for the hydrothermal products of (A) La ¼ LaeDy and (B) Ln ¼ HoeLu and Y. The insets show the local coordinations of Mo and MoeO bond lengths.
significantly more intense in Fig. 3B, which further confirmed the
existence of hydration water in the orthorhombic structure.
The results of electron microscopy for the series of double mo-
lybdates, which can be classified into three groups, are shown in
Fig. 4. The first group (Ln ¼ LaeSm) is in the form of irregular
blocks, whose thickness gradually decreases with decreasing size of
Ln3þ. This could be due to gradually more distortion of the crystal
structure along the c-axis, as seen from the much faster decreasing
c than a parameters (Fig. S3), which limits crystal growth along the
[001] direction. The second group (Ln ¼ EueDy) assumes spindle-
like morphologies, whose uniformity and size tend to improve to-
ward a smaller Ln3þ. It was noticed from the XRD patterns (Fig. 1A)
that the (004)/(101) diffraction intensity ratios of Ln ¼ LaeSm
(~2.65 for La, 3.12 for Ce, 4.47 for Pr, 4.82 for Nd, 2.58 for Sm) are
significantly larger than those of Ln ¼ EueDy (~1.86 for Eu, 1.45 for
Gd, 1.14 for Tb, 1.57 for Dy), which suggests that the former and
latter series have more exposed {001} and {101} facets, respectively
(Table S3). This is consistent with the different morphologies of the
two series. The third group (HoeLu and Y) takes rhombic shapes,
with thicknesses of ~50 nm and edge-lengths of ~100e300 nm,
which was confirmed by TEM analysis of the Lu sample. The
selected area electron diffraction of the Lu sample (SAED, the last
part of Fig. 4) yielded spots having interplanar spacings (d) of ~2.97,
2.09 and 1.70 Å, which are in close vicinity to the ~3.00, 2.09, and
1.67 Å derived via Rietveld structure refinement for the (111), (021)
and (130) planes (Table S4). The (111) and (021) planes were
observed in the SAED pattern to have a dihedral angle of ~89.3ꢀ,
which is also close to the calculated value of ~89.9ꢀ. The well cor-
responding results of SAED and Rietveld refinement further
confirmed the structure proposed for NaLnMo2O8$2H2O
(Ln ¼ HoeLu and Y).
was produced under pH ¼ 4 and 5 (Fig. 5A), whose crystallites
(Fig. 5C (a,b)) are in the forms of spindles (axis length ~9 m) and
blocks (~5
m), respectively. The pH ¼ 7 and 8 products are also
m
m
tetragonal NaY(MoO4)2, but they show much weaker XRD peaks
(Fig. 5A) and assume irregular sheet-like morphologies (Fig. 5C
(d,e)). Noteworthy is that the pH ¼ 6 product was crystallized as
orthorhombic NaYMo2O8$2H2O, whose crystallites are rhombic in
shape (Fig. 5C (c)). The XRD patterns of the Y samples synthesized
under pH ¼ 6 but at different MoO24ꢁ/Y3þ molar ratios are shown in
Fig. 5B. It is seen that the MoO24ꢁ/Y3þ ¼ 2 and 6 products are
tetragonal NaY(MoO4)2, whose crystallites are irregular sheets
(edge-lengths ~300e600 nm, Fig. 5C (f)) and spindles (axis length
up to ~14
m
m, Fig. 5C (j)), respectively. The MoO24ꢁ/Y3þ ¼ 5 product,
on the other hand, is the orthorhombic NaYMo2O8$2H2O product
consisting of rhombic crystallites (Fig. 5C (i)). Mixtures of the
tetragonal and orthorhombic phases were resulted at MoO42ꢁ
/
Y3þ ¼ 3 and 4, as also proved by the corresponding FE-SEM images,
which showed two types of crystallite morphologies (Fig. 5C (g, h)).
The above results thus suggest that the tetragonal/orthorhombic
phase preference may critically be influenced by the complicated
solution chemistry of Y and molybdate ions. It is known that
polycondensation of molybdate anions readily takes place under an
acidic condition to produce isopolyacid ions, and the degree of
polycondensation increases with decreasing solution pH [32]. It
was reported that monomeric MoO24ꢁ is the dominating species at
6ꢁ
pH ꢂ 6.5, while various polymolybdate anions, such as Mo7O and
24
4ꢁ
Mo8O , may be generated in the pH range of ~4e6.5 [32]. The
26
situation can be further complicated by the concentration of the
Na2MoO4 reactant, since it may shift reaction equilibria and alter
the relative contents of different molybdate species. Meanwhile,
Y3þ was expected to undergo extensive hydration and hydrolysis to
form [Y(OH)x(H2O)y]3ꢁx complex ion, owing to its small ionic size
and high charge, and the amount of hydroxyls in the complex
would increase with increasing solution pH [33]. The composition
(phase structure) of the hydrothermal product can thus be deter-
mined by the chemical reactions among the aforementioned
molybdate and Y species, whose exact types and relative contents
are affected by the conditions of hydrothermal reaction (pH and
MoO24ꢁ/Y3þ molar ratio). It is also noteworthy that, to minimize
system energy, the product of a chemical reaction under equilib-
rium is always the one having the lowest solubility product, which
Yttrium (r ¼ 1.019 Å), having an ionic size close to Ho3þ
(r ¼ 1.015 Å) [24], was identified to be the boundary element for
tetragonal NaLn(MoO4)2 and orthorhombic NaLnMo2O8$2H2O to be
formed. Considering that solution pH and MoO24ꢁ/Ln3þ molar ratio
would remarkably affect crystal structure and product morphology
[4,30,31], the additional experiments were thus performed to
evaluate the phase and morphology evolution of Y product.
The XRD patterns of the Y samples obtained under varying so-
lution pH but the same MoO42ꢁ/Y3þ molar ratio of 5:1 are presented
in Fig. 5A. It is seen that well crystallized tetragonal NaY(MoO4)2

Z. Xu et al. / Journal of Alloys and Compounds 830 (2020) 154676
5
Fig. 4. Morphologies of the hydrothermal products, and the results of TEM analyses of the Lu sample.
also varies with reaction condition but is unknown for the NaY(-
3.2. Transformation of NaLnMo2O8·2H2O into NaLn(MoO4)2 via
calcination
MoO4)2 and NaYMo2O8$2H2O compounds. While the exact mech-
anism is yet needed to be clarified, there is no doubt to say that the
phase structure and chemical composition of the final Y product is a
result of kinetic and thermodynamic competition of chemical
reactions.
As inferred from their chemical formula, the series of NaLn-
Mo2O8$2H2O compounds would transform to NaLn(MoO4)2 via
simple dehydration upon calcination. The course of phase evolution

6
Z. Xu et al. / Journal of Alloys and Compounds 830 (2020) 154676
Fig. 5. XRD patterns for the Y products obtained under (A) different pH values and MoO24ꢁ/Y3þ ¼ 5 molar ratio and (B) different MoO24ꢁ/Y3þ molar ratios and pH ¼ 6. Part (C) shows
the FE-SEM morphologies of the corresponding products.
was investigated with Ln ¼ Lu for example, and the results are
shown in Fig. 6. It can be observed that, while the 200 ꢀC product is
a mixture of NaLuMo2O8$2H2O and NaLu(MoO4)2, the calcination at
300 ꢀC has completely converted the hydration phase to tetragonal
NaLu(MoO4)2. TG analysis (Fig. S4) yields a total weight loss of
~6.7% up to ~400 ꢀC for the hydrated Lu sample and almost all the
weight loss occurred up to ~170 ꢀC. The number of hydration was
assayed to be ~2.32 from the total weight loss, which well conforms
to the NaLuMo2O8$2H2O formula. TG analysis of the NaLa(MoO4)2
sample only yielded a negligible weight loss of ~0.68%, which could
be due to evaporation of surface absorbed water molecules. This
further confirmed that NaLuMo2O8$2H2O underwent more dehy-
dration upon being heated.
To better illustrate the effects of lanthanide contraction on
structure, the calcination was performed for all the hydrothermal
products at the same temperature of 500 ꢀC for 2 h. From the XRD
patterns shown in Fig. 7A, it is clear that all the calcination products
are phase-pure tetragonal NaLn(MoO4)2 but the diffraction peaks
steadily shift to higher angles with decreasing ionic radius of Ln3þ
,
as manifested in Fig. 7B with the (112) diffraction for example. The
calculated cell parameters (a, c) and cell volume (V) linearly
decrease with decreasing radius of Ln3þ (Fig. 7C), in compliance
with lanthanide contraction. In the FE-SEM observation, it is found
that the larger crystallites of Ln ¼ LaeDy basically maintained their
original morphologies while the smaller ones of Ln ¼ HoeLu and Y
underwent substantially deformation (Fig. S5) during calcination.
3.3. Luminescence properties of the NaLn(MoO4)2-Based phosphors
The excitation (PLE) and emission (PL) spectra of the Eu3þ doped
samples before annealing at 500 ꢀC are given in Fig. 8A. It is clear
that the excitation spectra consist of a strong and broad band
ranging from 200 to 350 nm and a series of sharp lines in the
350e500 nm spectral region in each case. The broad band can be
deconvoluted by Gaussian fitting into two components that
correspond to OeEu (CTB1; ~260e284 nm) and OeMo (CTB2;
~303e321 nm) charge transfer [2,34]. The sharp lines are arising
from the intra-4f6 transitions of Eu3þ, which correspond to exci-
tation of electrons from 7F0 ground state to the higher lying energy
levels of 5D4 (~362 nm), 5G2 (~381 nm), 5L6 (~395 nm), 5D3
(~415 nm), and 5D2 (~465 nm) [35]. The CTB2 bands of the Ln ¼ Y
and Lu samples are red shifted by ~20 nm when compared with
those of the Ln ¼ La and Gd samples, which is primarily owing to
Fig. 6. XRD patterns of NaLuMo O $2H O and its calcination products. The symbols
indicate the NaLuMo2O8$2H2O phase.
A
2
8
2

Z. Xu et al. / Journal of Alloys and Compounds 830 (2020) 154676
7
Fig. 7. XRD patterns (A), enlarged view of the 2
q
¼ 27.5e30ꢀ region for the (112) diffraction (B), and correlation of lattice constants (a, c) and cell volume (V) with the radius of Ln3þ
(8-fold coordination) for the full series of NaLn(MoO4)2 calcined at 500 ꢀC for 2 h (C).
Fig. 8. The excitation (PLE) and emission (PL) spectra of the Eu3þ-doped samples before (A) and after (B) calcination at 500 ꢀC for 2 h. The insets in Fig. 8A show deconvolution of the
250e350 nm bands via Gaussian fitting.
the different coordination environments of Mo6þ. In this work, the
Mo6þ ion is four-fold coordinated in tetragonal NaLn(MoO4)2
(Ln ¼ La, Gd) while five-fold coordinated in orthorhombic NaLn-
Mo2O8$2H2O (Ln ¼ Y, Lu). It was reported that OeMo CTB may red-
shift from ~260 to 295 to 300e350 and then to 375e400 nm with
the coordination number of Mo increasing from 4 to 5 and then to
6, owing to gradually stronger crystal field splitting of the Mo6þ 5d
energy level [36,37]. The position of OeEu charge transfer (CTB1),
on the other hand, is affected by the length and covalency of OeEu
bond, and would red shift with increasing bond length/covalency.
The excitation and emission spectra of the Na(Ln0.95Eu0.05)(MoO4)2
samples calcined at 500 ꢀC are shown in Fig. 8B. It is seen that the
CT bands are all centered at ~280 nm, irrespective of the type of Ln,
which conforms to the same crystal structure and coordination
environment of Mo6þ (Eu3þ). The merging of CTB1 and CTB2 sub-
bands could be due to better crystallization by calcination, since
the OeEu and OeMo polyhedrons would be relatively isolated in
the direct hydrothermal products by low crystallinity. The intensity
of CTB follows the order Gd > La > Y > Lu, and the extraordinarily
strong CTB of the Gd sample is primarily owing to mixing in of the
8S7/2 //6PJ (~275 nm) intra-4f7 excitation of Gd3þ in the host. For
the products of the other three Ln elements, the excitation intensity
was observed to decrease following the order La > Y > Lu. By
considering Ln-O-Eu(Mo) moiety, the results could be due to the
fact that the electronegativity of Ln (the ability to attract O-2p
electrons) increases in the order La < Y < Lu [24], which decreases
the probability (intensity) of OeEu(Mo) charge transfer.
The samples before and after 500 ꢀC calcination all exhibited
emissions in the 500e750 nm spectral region, which correspond to
transitions from the excited 5DJ (J ¼ 0,1) levels to 7FJ (J ¼ 1, 2, 3, 4)
ground states, including 5D1
/ /
7F1 (~538 nm), 5D0 7F1
(~595 nm), 5D0 / 7F2 (~615 nm), 5D0 / 7F3 (~651 nm), and 5D0 /
7F4 (~701 nm) [38e40]. The similar emissions are due to the
shielding effect of 5s25p6 electrons, which makes the above intra-
4f6 transitions insensitive to crystal field. The 615 nm (5D0
/
7F2)
red emission is dominantly strong in each case, since Eu3þ resides

8
Z. Xu et al. / Journal of Alloys and Compounds 830 (2020) 154676
at the low symmetric D2h site in orthorhombic NaLnMo2O8$2H2O
and S4 site in tetragonal NaLn(MoO4)2 [41]. In addition, the emis-
sion intensity follows the order observed for CTB either before or
after calcination. For the calcined powders, the 615 nm emission
intensity of Ln ¼ Gd is ~1.78, 2.44, 4.36 times those of Ln ¼ La, Y, and
Lu, respectively. Calcination greatly improved Eu3þ luminescence,
owing to crystallite growth and structure perfection, and the
increment of ~64% was found for the 615 nm emission of the
Ln ¼ Gd sample.
ratio (I658/I543; Table S6). The phosphor showed a bright red
emission under 2 W of laser excitation, as shown by the inset
photograph (Fig. 9A). The intensity (Iem) of UC luminescence is
proportional to the nth power of the pumping laser power (P)
under unsaturation, i.e., Iem f Pn, where n represents the number of
pumping photons needed to produce one emitted photon. The
double logarithmic transformation of the experimental data is
shown in Fig. 9B, and the n values were determined from the slopes
of linear fitting to be ~2.66 0.09 and 2.46 0.09 for the 543 green
and 658 nm red emissions, respectively.
NaLn(MoO4)2 is a suitable host lattice for upconversion (UC)
luminescence because of its tꢁh1ermal stability and relatively low
phonon energy (~800 - 890 cm ) [42e44]. The XRD patterns of the
NaLu(MoO4)2:0.1 Yb3þ,0.02RE3þ UC phosphors (RE ¼ Ho, Er)
calcined at 500 ꢀC are shown in Fig. S6, where it is evident that the
patterns are similar to each other and match well with that of
tetragonal NaLu(MoO4)2 in the standard diffraction file (JCPDS No.
54e0896). FE-SEM observation (Fig. S6) showed that the products
tend to have the crystallite morphology of NaLuMo2O8$2H2O
(Fig. 4). In Fig. 9A, the UC emission spectra are shown for
NaLu(MoO4)2:0.1 Yb3þ,0.02Ho3þ under different power of 978 nm
laser excitation. The spectra is composed of a negligibly weak blue
band (~475 nm), a relatively strong green band (~543 nm) and a
double-split strong red band (~658 nm), in each case, which
correspond to transitions from the 5F3, 5F4/5S2 and 5F5 excited states
to 5I8 ground state of Ho3þ, respectively [45e47]. Raising the
excitation power from 0.6 to 2.0 W steadily improved the three
groups of emissions, but the color of luminescence gradually
moved from red [color coordinates around (0.50, 0.44)] to yellow
[color coordinates around (0.50, 0.47)] (Table S5 and Fig. S7). This is
in accordance with the successively decreasing red/green intensity
The UC emission spectra of NaLu(MoO4)2:0.1 Yb3þ,0.02Er3þ
under varying excitation power are exhibited in Fig. 9C. It is clear
that each spectrum contains two groups of strong green lumines-
cence (peaked at ~532 nm and 553 nm, respectively) and one group
of weak red emission (centered at ~657 nm) originate from the
4
2H11/2/4S3/2 //4I15/2 and F9/2
/
4I15/2 transitions of Er3þ, respec-
tively [45,48,49]. Increasing excitation power led to gradually
higher intensity for each group of the bands (Fig. 9C), together with
gradually larger I532/I657 and I553/I657 green/red intensity ratios
(Table S6). This accounts for the changing color coordinates
(Table S5) and shifting of the luminescence color toward the green
corner of the International de L’Eclairage (CIE) chromaticity dia-
gram (Fig. S7). In further analysis, it is found that, with increasing
excitation power, the 532 nm emission gained intensity much
faster than the 553 nm one (Table S6) and this is due to the fact that
2
the H11/2/4S3/2 energy levels of Er3þ are thermally coupled and,
therefore, some of the 4S3/2 electrons may jump to the higher lying
2H11/2 state via thermal activation. The inset picture in Fig. 9C
clearly demonstrated a strong green luminescence of the phosphor
under 2 W of laser excitation. Double logarithmic analysis of the
Fig. 9. UC luminescence spectra (A, C) and double logarithmic plots of UC emission intensity against pump power (B, D) for NaLu(MoO4)2:0.1 Yb3þ,0.02Ho3þ (A, B) and
NaLu(MoO4)2:0.1 Yb3þ,0.02Er3þ (C, D). The insets in (A) and (C) are digital photographs for the appearance of UC luminescence.

Z. Xu et al. / Journal of Alloys and Compounds 830 (2020) 154676
9
experimental data yielded n values of ~2.92 0.10, 2.55 0.08 and
levels of Er3þ are preferentially populated.
2.42 0.09 for the 532, 553 and 657 nm emissions (Fig. 9D).
Though the n values determined for Ho3þ and Er3þ lumines-
cence fall in between 2 and 3, a two-photon mechanism was
adopted in this work to explain the process of UC by considering
possible transmission loss of laser power. The proposed energy
level diagrams and UC processes for Yb3þ/Ho3þ and Yb3þ/Er3þ are
shown in Fig. 10, where the Yb3þ-MoO42- dimer synergistically
sensitizes Ho3þ and Er3þ luminescence through absorbing excita-
tion energy via ground state absorption (GSA) and excited state
4. Conclusions
With lanthanide (Ln ¼ LaeLu, except for Pm; including Y) nitrate
and di-sodium molybdate as reactants, the hydrothermal reaction
at 180 ꢀC for 24 h under MoO24ꢁ/Ln3þ ¼ 5 molar ratio and pH ¼ 6
produced tetragonal NaLn(MoO4)2 and orthorhombic NaLn-
Mo2O8$2H2O phases for the lager and smaller Ln3þ of Ln ¼ LaeDy
and Ln ¼ HoeLu and Y, respectively, with Y being the demarcation
point of the two structures. The series of hydrated compounds can
be converted to the tetragonal phase by calcination at the low
temperature of 300 ꢀC, and the full series of NaLn(MoO4)2 calcined
at 500 ꢀC showed almost linearly decreasing cell parameters and
cell volume toward a smaller Ln3þ. The phase preference of the Y
product was found to depend on pH and MoO24ꢁ/Y3þ molar ratio in
reaction solution. The photoluminescence investigation showed
that NaLn(MoO4)2:Eu (Ln ¼ La, Gd, Y, Lu) can be effectively excited
by ~280 nm UV light via charge transfer for dominantly strong red
emission at ~615 nm, whose excitation/emission intensities in-
crease in the order Lu < Y < La < Gd. The upconversion lumines-
cence of Yb3þ/Ho3þ and Yb3þ/Er3þ pairs in NaLu(MoO4)2 was found
to be dominated by ~650 nm red and ~530e550 nm green bands,
respectively, and the UC luminescence can be rationalized with a
two-photon process involving Yb3þ-MoO42- dimer sensitization.
absorption (ESA) [50]. In the |2F7/2
|2F7/2
,
1A1>, |2F5/2
,
1A1>, |2F7/2
,
3T1>,
2
2
,
3T2>, |2F7/2, 1T1> and |2F7/2, 1T2> denotations, F7/2 and F5/2
stand for the ground and excited states of Yb3þ within the 4f13 shell
while 1A1 and 3T1, 3T12, 1T1 and 1T2 denote the ground state and
successively higher excited states of Mo6þ 5d energy states,
respectively. Under the 978 nm laser excitation, the Yb3þ-MoO42-
dimer absorbs the energy of the first laser photon, with which the
|2F7/2, 1A1> ground state electrons are raised to the |2F5/2, 1A1> in-
termediate state via GSA. With the energy of the second laser
photon, the electrons can be further promoted to the |2F7/2
,
1T1>
excited state through ESA, followed by non-radiative relaxation
(NR) to the |2F7/2
,
3T2> level. It is worth noting that the excitation
energy at |2F7/2, 3T2> is more likely to relax to the 5F3 level of Ho3þ
and the 4F7/2 level of Er3þ by high-excited energy transfer (HESET)
[50e53]. In the case of Ho3þ, the energy transferred to the 5F3 level
may decay to the 5F4/5S2 and 5F5 states via NR, from which the
green (~543 nm; 5F4/5S2
/ /
5I8) and red (~658 nm; 5F5 5I8)
Declaration of competing interest
emissions can then be produced. The negligibly weak ~475 nm blue
emission (5F3 5I8) indicates that the above NR processes are
significant. In addition, 5F3 5I8 5F5 5I7 cross relaxation (CR)
/
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
þ
/
þ
likely occurs due to small energy mismatch, which improves pop-
ulation of the 5F5 level, and, therefore, leads to stronger red than
green emission. In the case of Er3þ, the energy transferred to the
4F7/2 level may relax via NR to the 2H11/2 and 4S3/2 states, followed
CRediT authorship contribution statement
4
by radiative relaxation of most of the excitation energy to I15/2 to
Zhixin Xu: Investigation, Data curation, Funding acquisition,
Writing - original draft. Siyuan Li: Formal analysis. Qinghong
Meng: Investigation. Xuejiao Wang: Methodology, Funding
acquisition. Qi Zhu: Resources, Funding acquisition. Ji-Guang Li:
Conceptualization, Writing - review & editing.
produce strong green emission. The weak red emission at ~657 nm
4
implies that only a small part of the S3/2 energy can further relax
via NR to the 4F9/2 level. The rapidly increasing green/red intensity
ratio (from ~12.7 to 23.5 for I532/I657 and from ~12.1 to 14.2 for I553
/
I657) along with the excitation power increasing from 0.6 to 2.0 W
2
4
2
(Table S6) implies that the H11/2 and S3/2 (particularly H11/2
)
Acknowledgements
This work was partially supported in part by the Fundamental
Research Funds for the Central Universities (Grant N180206002)
and the National Natural Science Foundation of China (Grant No.
51672039, 51702020 and 51972047). Z.X. Xu acknowledges finan-
cial support from the China Scholarship Council (Grant
201806080123) for her Ph. D. study in Japan.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jallcom.2020.154676.
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