Disorder-Induced Breaking of the Local Inversion Symmetry in

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Disorder-Induced Breaking of the Local Inversion Symmetry in Rhombohedral Pyrochlores M2La3Sb3O14 (M = Mg or Ca): A Structural and Spectroscopic Investigation Fabio Piccinelli,*,† Irene Carrasco,‡ Chong-Geng Ma,*,†,§ Alok M. Srivastava,∥ and Marco Bettinelli† †

Luminescent Materials Laboratory, DB and INSTM, UdR Verona, Università di Verona, Strada Le Grazie 15, 37134 Verona, Italy Advanced Technology Institute, Department of Electrical and Electronic Engineering, University of Surrey, Guildford GU2 7XH, United Kingdom § College of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, PR China ∥ GE Global Research, One Research Circle, Niskayuna, New York 12309, United States

Downloaded via UNIV OF SOUTH DAKOTA on July 20, 2018 at 08:32:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: A detailed investigation of the overall crystal structure, and in particular of the local structure around the cations in M2La3Sb3O14 (M = Mg, Ca) was accomplished using X-ray diffraction, steady state luminescence spectroscopy and decay kinetics, and state of the art density functional calculations. The computational tool was also used to investigate the structure of Mn2La3Sb3O14. The Eu3+ dopant ion was employed as an optical probe of the local symmetry at the cationic sites. The use of these complementary techniques shows that the antimonates under investigation belong to the rhombohedral pyrochlore family with space group R3̅m (No. 166), but while Mg2La3Sb3O14 and Mn2La3Sb3O14 show an ordered cationic configuration, the Ca2+ and La3+ of Ca2La3Sb3O14 are disordered because of their similar ionic radii. In both the Mg- and the Ca-based compounds, the Eu3+ ions formally occupy centrosymmetric sites, but in the case of Ca2La3Sb3O14 the presence of disorder in the outer coordination spheres removes the local inversion symmetry in these sites. This has a strong influence on the Eu3+ luminescence spectrum and on the radiative decay rate of the 5D0 emitting level.

■

INTRODUCTION

reasonable, since the hexagonal bipyramid motif characteristic of the weberite structure is not present in Ca2Ln3Sb3O14 as not all oxygen atoms are part of the corner-linked metal−oxygen framework. Moreover, the previous choice of I2/m (a subgroup of R3̅m) was possibly accidental as any rhombohedral cell may have a derivative monoclinic subcell6 with identical d-values, which for the case of Ca2Ln3Sb3O14 are given in the Figure 6 of ref 5. Although the qualitative analysis given here can partially eliminate the discrepancies in the crystal structure description of this type of materials, a quantitative analysis, such as the comparison of the total energies per formula unit of the two proposed crystal phases, is still needed for the sake of scientific rigor. Apart from the apparently solved issue of the space group choice, we also wondered whether the crystallographic studies of Ca2Ln3Sb3O14, considering the presence of disorder between Ln3+ and Ca2+ ions, can properly reproduce the real local site environments of Ln3+ ions. A quick inspection of the crystal structure of Ca2Ln3Sb3O14 reveals that all three

The nature of the space group of Ca2Ln3Sb3O14 (hereafter Ln denotes any element of the lanthanide series and yttrium) has been the subject of many discussions among scientists. Based on X-ray powder diffraction data, Burchard and Rüdorff1 proposed a body centered orthorhombic cell for the antimonates containing La, Nd, Sm, and Gd and a primitive tetragonal unit cell for Dy, Yb, Lu, and Y but did not give any additional details on the structure. In 2007, Au et al. refined for the first time the structure of Ca2Ln3Sb3O14 on the basis of the monoclinic space group I2/m,2 a subgroup of the space group Imma typically used for the description of the orthorhombic weberite-type structure.3 Subsequently, the experimentally refined crystal structure of Ca2La3Sb3O14 was taken as the initial model for a first-principles geometry optimization using the local minimum search algorithm, thus further determining the theoretical investigation of several electronic properties at a density-functional-theory level.4 This seemed to be the end of the story. However, in 2015, the space group I2/m was revised by Fu and IJdo5 as R3̅m, which is not weberite-like but belongs to the family of rhombohedrally distorted pyrochlores. From a structural point of view, such an assignment looks more © XXXX American Chemical Society

Received: May 8, 2018

A

DOI: 10.1021/acs.inorgchem.8b01261 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. (a) Crystal structure of rhombohedral pyrochlores AA′3BB′3O14. (b) Oxygen coordination environments of the four sites A, A′, B, and B′. A/B = M, A′ = La, and B′ = Sb for the ordered M2La3Sb3O14 compounds (for example, M = Mg and Mn), whereas the disorder occurs between M and La atoms on the three sites A, B, and A′ for the disordered cases (such as the Ca analogues).

available Ln3+ (or Ca2+) sites are centrosymmetric both in the I2/m and R3̅m space groups. However, the spectroscopic investigation by Srivastava et al.7 using Eu3+ as a structural probe in Ca2La3Sb3O14 has clearly pointed out that the observed typical 5D0 emission originates from Eu3+ ions located in at least three noncentrosymmetric sites, presumably the ones occupied by La3+, on the basis of charge equivalence. Such a contradiction motivated us to explore whether the disorder between Ln3+ and Ca2+ ions leads to the breaking of the local inversion symmetry of Ln3+ sites in Ca2Ln3Sb3O14. For this reason, fully ordered rhombohedral pyrochlores are the simplest cases to consider to check the present speculation and to perform a comparative spectroscopic study with the one reported by Srivastava et al.7 Recently, our attention has been drawn to a new type of ordered rhombohedral pyrochlores with the general formula, Mg2Ln3Sb3O14.8 Such compounds are isostructural with Ca2Ln3Sb3O14, but the main difference with the Ca-based analogues is the full cationic ordering, as suggested by their structural formula AA′3BB′3O14 in which A and A′ are 8-fold and B and B′ are 6-fold coordinated (see Figure 1). In Mg2Ln3Sb3O14, the sites A and B are fully occupied by Mg while A′ is fully occupied by Ln and B′ by Sb. This cation ordering is driven by the larger ionic radius difference between the Mg2+ and Ln3+ cations. Therefore, in the Mg-based structure, the cationic sites are fully ordered, at variance with the situation in Ca-based material. As a consequence, upon introduction of Eu3+ ions into the Ln3+ sites of the Mg-based compound, the Eu3+ hypersensitive transition 5D0 → 7F2 is expected to be suppressed due to the fully centrosymmetric environment of the Ln3+ sites caused by the strict ordering of the host cations. In contrast, the disorder between Ln3+ and Ca2+ ions with different distribution ratios in the three sites A, B, and A′ can break the local inversion symmetry of Ln3+ sites in Ca2Ln3Sb3O14 and therefore induce a significant increase of intensity of the electric−dipole transitions 5D0 → 7F0 and 5D0 → 7F2 of Eu3+. In other words, the presence of the so-called “center of symmetry” in the crystal structure is actually triggered by the use of the virtual or “mixed” cationic species “Ca/Ln”,9 and thus it should be just regarded as a statistically averaged effect. Therefore, such a comparison between the structure and spectroscopy of the ordered and disordered compounds doped with Eu3+ ions should be interesting for readers working on the topic of luminescence intensity enhancement of lanthanide ions in solids. Moreover, it is also desirable to establish the local coordination environment

of the substitutional Ln3+ sites occupied by Eu3+ ions by a schematic representation to persuasively confirm the local breaking of the inversion symmetry in Ca2Ln3Sb3O14. In the present contribution, we focused our attention on the ordered and disordered rhombohedral pyrochlores Mg2La3Sb3O14 and Ca2La3Sb3O14 to verify the breaking of the local inversion symmetry for La3+ on moving from the former to the latter; the luminescence spectroscopic properties of 1% Eu3+ doped Mg2La3Sb3O14 and 1% Eu3+ doped Ca2La3Sb3O14 are compared and discussed. Moreover, firstprinciples calculations were performed to confirm the structural evidence stemming from X-ray diffraction (XRD) and to elucidate the structure−spectroscopy relationship of Eu3+-doped materials. We believe that the results presented here can provide quantitative answers to the three following questions: (1) Why is the Mg2La3Sb3O14 host ordered while Ca2La3Sb3O14 is not? (2) Which one of I2/m and R3̅m is a more reasonable space group for the Ca2La3Sb3O14 host? (3) How does the disorder between La3+ and Ca2+ ions in Ca2La3Sb3O14 induce the breaking of the local inversion symmetry of La3+ sites? We also provide a comparative study pertaining to computational results obtained on Mg2La3Sb3O14 and the manganese analogue Mn2La3Sb3O14.

■

EXPERIMENTAL SECTION

Materials and Synthesis. Crystalline samples of Mg2La3Sb3O14 and 1 mol % Eu3+ doped Mg2La3Sb3O14 (replacing La3+) were prepared by solid state reaction at high temperature. MgO (99%), Sb2O3 (99.999%), La2O3 (99.99%), and Eu2O3 (99.99%) were thoroughly mixed and pressed into pellets under a pressure of 10 tons. The samples underwent two heat treatments under air atmosphere at 800 and 1300 °C for 12 and 18 h, respectively, with intermediate grindings. The synthesis of Ca2La3Sb3O14 doped with 1 mol % of Eu3+(substituting for La3+) has been performed following a procedure already described in the literature.7 Structural Characterization. XRD patterns were measured with a Thermo ARL X’TRA powder diffractometer, operating in the Bragg−Brentano geometry and equipped with a Cu-anode X-ray source (Kα, λ = 1.5418 Å), using a Peltier Si(Li) cooled solid state detector. The patterns were collected with a scan rate of 0.002°/s in the 16°−120° 2θ range. Polycrystalline Mg2La3Sb3O14 sample were ground in a mortar and then put in a side-loading sample holder for the data collection. The General Structure Analysis System (GSAS) program was employed for the Rietveld refinement calculations.10 The instrumental X-ray peak profile functions and the sample displacement (SHFT variable) were determined by Rietveld refinement on the diffraction pattern of the LaB6 powder standard reference material (NIST 660C). B


Article

Inorganic Chemistry The reference structural model exploited in the Rietveld calculation was that determined by Sanders et al.8 of the ordered rhombohedral pyrochlore Mg2La3Sb3O14. The following structural refinement strategy has been performed: (i) refinement of the background functions (shifted Chebyschev), scale factor, and cell parameters; (ii) refinement of the fractional atomic coordinates for the oxygen atoms; (iii) refinement of the isotropic thermal parameter (Uiso) for La and Sb ions; (iv) refinement of the isotropic thermal parameter (Uiso) for Mg ion and oxygen atoms (we do not observe significant improvement of the refinement employing anisotropic thermal parameters); (v) global refinement of all structural variables mentioned above. In each step of the Rietveld calculation and the GU, GV, GW, LX, LY, and asym profile terms of the Pseudo-Voigt profile function no. 2, included in the GSAS program, were refined. CCDC 1843048 contains the supplementary crystallographic data for Mg2La3Sb3O14 structure discussed in this paper. These data can be obtained free of charge by contacting FIZ Karlsruhe at +497247808666 (fax) or crysdata@fiz-karlsruhe.de (e-mail). Spectroscopic Characterization. Room temperature luminescence spectra and decay curves were measured with a Fluorolog 3 (Horiba-Jobin Yvon) spectrofluorometer, equipped with a Xe lamp, a double excitation monochromator, a single emission monochromator (mod. HR320), and a photomultiplier in photon counting mode for the detection of the emitted signal. All the spectra were corrected for the spectral response of the setup. DFT Calculations. The first-principles calculations of the structural and electronic properties of M2La3Sb3O14 (M = Mg, Ca, and Mn) were performed by using the periodic ab initio CRYSTAL14 code based on the linear combination of atomic orbitals method with the local Gaussian-type basis sets (BSs).11 The closed-shell calculation form was applied for M2La3Sb3O14 (M = Mg and Ca), whereas the spin-polarized one was considered for Mn2La3Sb3O14 due to the 3d open-shell character of Mn2+ ions. The hybrid exchange-correlation functional WC1PBE12 was used in this work in order to yield a good description of the electronic properties of solids.13 The all-electron BSs 86-411d41G, 86-511d21G, 8-511d1G, and 8-411d1G were chosen for manganese,14 calcium,15 magnesium,16 and oxygen17 atoms, respectively. For heavy antimony18 and lanthanum19 atoms, the small-core relativistic effective core pseudopotentials of the Stuttgart/Cologne group and their related valence BSs were adopted, and the diffuse functions with exponents less than 0.1 bohr−2 in those valence BSs were removed to avoid numerical catastrophes. In all the self-consistent field (SCF) calculations, we employed an 8 × 8 × 8 kpoint mesh in the Brillouin zone based on the Monkhorst−Pack scheme20 and set the truncation criteria of the bielectronic integrals (Coulomb and HF exchange series) as 8, 8, 8, 8, and 16. A predefined “extra extra large” pruned DFT integration grid and the energy convergence threshold with 10−9 hartree for the SCF iterations were chosen for the sake of high accuracy. The convergence thresholds of the root-mean-square of the gradient and nuclear displacement for the geometry optimization were set to 0.00006 hartree/bohr and 0.00012 bohr, respectively. For the Mn-based compound, the Anderson mixing scheme with 60% mixing was applied to improve the SCF convergence speed, and the spin value of each Mn2+ ion was locked to 5/2 in the first few SCF cycles to obtain a final ferromagnetic solution.

types of kagome layers are schematically depicted in Figure 1b in order to get a closer view of the fine structure of each kagome layer. Following such understanding of the structural character of AA′3BB′3O14, one can find that in M2La3Sb3O14, the sites A and B, characterized by the point-group symmetry 3̅m (D3d), can be occupied by M2+ ions, while La3+ and Sb5+ ions are located, respectively, at the sites A′ and B′, characterized by the point-group symmetry 2/m (C2h), if strict order for each site is considered. The first example reported in the literature is the fully ordered compound Mn2La3Sb3O14 described by Fu and IJdo in 2014.21 After that, the prototype obtained from Mn2La3Sb3O14 was examined for the first time by taking into account the disorder between M2+ and La3+ ions on the three sites A, A′, and B and then successfully applied to the XRD refinements of the Ca-analogues.5 Recently Sanders et al. have performed the Rietveld structure refinement of Mg2La3Sb3O14 without considering disorder between Mg2+ and La3+ ions.8 From the point of view of structural chemistry, since in general the XRD technique is very sensitive to the electronic density at each crystal site, a significant difference is expected in the XRD data when considering the ordered and disordered configurations of cations with very different atomic numbers, such as Mg2+, Ca2+, and La3+. For this reason, the ordered and disordered structural models for Mg- and Ca-based antimonates, respectively, must be considered as the most reliable descriptions. In order to corroborate this statement, we performed a Rietveld refinement on the powder pattern of Mg2La3Sb3O14 by using a disordered La/Mg model, and as expected, the refinement factors wRp and Rp were found to be significantly higher than for the ordered model (21.05% and 14.22% against 12.47% and 8.92%, respectively). In addition, it is reasonable to assume that the smaller difference between the ionic radii of Ca2+ and La3+ with respect to Mg2+/Mn2+ and La3+ (as shown in Table 1) can result in a higher level of cation disorder. However, Table 1. Ionic Radii (in Å) of La3+, Ca2+, Mn2+, Mg2+, Cd2+, Hg2+, Co2+, and Zn2+ Ions for the Three Sites A, B, and A′a r(A, CN = 8) La3+ Ca2+ Mn2+ Mg2+ Cd2+ Hg2+ Co2+ Zn2+

1.16 1.12 0.96 0.89 1.10 1.14 0.90 0.90

(−0.04) (−0.20) (−0.27) (−0.06) (−0.02) (−0.26) (−0.26)

r(B, CN = 6) 1.032 1.00 (−0.032) 0.83 (−0.202) 0.72 (−0.312) 0.95 (−0.082) 1.02 (−0.012) 0.745 (−0.287) 0.74 (−0.292)

r(A′, CN = 8) 1.16 1.12 0.96 0.89 1.10 1.14 0.90 0.90

(−0.04) (−0.2) (−0.27) (−0.06) (−0.02) (−0.26) (−0.26)

a

Data were taken from ref 22. The abbreviation CN stands for the site coordination number. The values in parentheses are the ionic radius difference with respect to La3+.

■

RESULTS AND DISCUSSION Structural Determination of M2La3Sb3O14 (M = Mg and Ca). The crystal structure of pyrochlores AA′3BB′3O14 with the rhombohedral space group (R3̅m) consists of the alternative stacking of the two kagome layers A′3B and B′3A, as shown in Figure 1a. A further inspection of these layers reveals that each one is composed of a series of hexagonal rings formed by A′O8 bipyramids or B′O6 octahedra, and the centers of those rings are filled by the second cation unit of each layer, that is, the BO 6 octahedra or AO 8 bipyramids. The constitutional units AO8, A′O8, BO6, and B′O6 for the two

persuasive theoretical evidence is still needed in order to give a quantitative criterion on whether to take into account the disorder between M2+ and La3+ ions when using the rhombohedral pyrochlore model for the Rietveld structure refinements of other M2La3Sb3O14 compounds. Therefore, to evaluate the disorder levels in the Ca- and Mg-based compounds, we calculated by the first-principles methods the energy costs for the exchange of a pair of ions between La3+ and M2+ sites in the rhombohedral primitive cell [with one chemical formula of M2La3Sb3O14 (M = Ca, Mn, and Mg)] and compared the calculation results for the Ca- and Mg-based C


Article

Inorganic Chemistry

that both Cd2La3Sb3O14 and Hg2La3Sb3O14 should be disordered due to very similar ionic radii of Cd2+ and Hg2+ as La3+, whereas there should be a strict ordering in Co2La3Sb3O14 and Zn2La3Sb3O14 due to much smaller ionic radii of Co2+ and Zn2+ than Mn2+.22 This is indeed the case, as confirmed by two papers in the literature.23,24 In the light of our theoretical understanding given above, we employed the structural model without taking into account the disorder between Mg2+ and La3+ ions to refine the measured XRD pattern of Mg2La3Sb3O14, as done by Sanders et al.8 The observed and fitted XRD patterns of Mg2La3Sb3O14 are shown in Figure 2. Inspection of this figure shows that there is a good

compounds with those for the fully ordered compound Mn2La3Sb3O14 based on the consideration that M2La3Sb3O14 should be ordered if its M−La exchange formation energy is bigger than that in Mn2La3Sb3O14. It can be easily found that there are ten structural configurations if the disorder between M2+ and La3+ ions is considered on the 5 M/La positions in one chemical formula of M2La3Sb3O14 compound. The symmetry analysis performed by the program module “CONFCNT” of CRYSTAL1411 reveals that they can be grouped into four symmetryindependent classes due to the nonequivalence between any two of the three sites A, B, and A′, as follows: M(A)M(B)La3(A′)Sb3O14, La(A)M(B)M(A′)La2(A′)Sb3O14, M(A)La(B)M(A′)La2(A′)Sb3O14 and La(A)La(B)M2(A′)La(A′)Sb3O14, and the degenerations within these classes are 1, 3, 3, and 3, respectively. The representative structural configurations of such four symmetry-independent classes were relaxed, and their calculated total energies and optimized structural data are reported in Tables 2 and S1−S3, Table 2. Calculated Total Energies (in eV) of the Representative Structural Configurations of Four Symmetry-Independent Classes for One Rhombohedral Primitive Cell of M2La3Sb3O14 (M = Mg, Ca, and Mn)a symmetry-independent class

M = Ca

M = Mn

M = Mg

M(A)M(B)La3(A′)Sb3O14 La(A)M(B)M(A′)La2(A′)Sb3O14 M(A)La(B)M(A′)La2(A′)Sb3O14 La(A)La(B)M2(A′)La(A′)Sb3O14

0 0.4871 1.3310 1.3640

0 0.9906 1.6255 2.3640

0 1.0186 1.8407 2.6437

Figure 2. Observed (crosses) and refined (continuous red line) powder patterns of Mg2La3Sb3O14. The observed − refined curve is shown at the bottom of the plot.

a

The total energies of the most stable forms M(A)M(B)La3(A′)Sb3O14 (M = Mg, Ca, and Mn) are set as zero as a reference.

respectively. Upon inspection of the results collected in Table 2, we can draw the following four conclusions: (1) the first class with the unique structural configuration, M(A)M(B)La3(A′)Sb3O14, is most stable; (2) the M−La exchange between the A and A′ sites is most favorable due to the lowest energy cost; (3) the energy cost for locating La3+ ions into the B site is about 2 times higher than that into the A site; (4) the M−La exchange formation energy of each symmetryindependent class increases along the “Ca → Mn → Mg” direction. Conclusion 1 suggests the reason why the M(A)M(B)La3(A′)Sb3O14 configuration can be adopted as starting structural model for the Rietveld refinement of other M2La3Sb3O14 compounds. Conclusions 2 and 3 are not surprising because the larger La3+ ions will prefer a ligand environment with high coordination number. They are also further confirmed by the refined structural model of Ca2La3Sb3O14 in which the La3+ occupancy on the A site is 40.3% but that on the B site is 5.9%.5 The final conclusion 4 clearly shows that all the sites of Mg2+ and La3+ ions in the structural model for the XRD refinement should be regarded as ordered, whereas consideration of disorder between Ca2+ and La3+ ions is indeed necessary for the XRD refinement of Ca2La3Sb3O14, since the energy cost of the M−La exchange between the A and A′ sites is bigger in Mg2La3Sb3O14 than in Mn2La3Sb3O14 and in the case of Ca2La3SbO14 it is about onehalf that for Mn/Mg−La. Moreover, the calculated trend of the M−La exchange energy cost in Table 2 is the same as can be deduced by structural chemistry consideration as given in Table 1. One could even use the values of the ionic radii of Cd2+, Hg2+, Co2+, and Zn2+ ions listed in Table 1 and predict

agreement between the observed and refined powder patterns. The refined lattice parameters and atomic positions are given in Table 3, and some selected bond distances are listed in Table 4. The first-principles geometry optimization of Mg2La3Sb3O14 was implemented using the structural data of Mn2La3Sb3O14,21 replacing Mn by Mg as an initial guess. The predicted structural parameters were collected and tabulated in Tables 3 and 4 for the sake of comparison. Inspection of Tables 3 and 4 shows that the prediction of the structural features is in very good agreement with the refined experimental data. Furthermore, our structural results presented are also very consistent with what was obtained by Sanders et al.8 For instance, we also detected short metal−oxygen distances for O1: Mg(2)−O(1) = 1.98 Å and La−O(1) = 2.38 Å and one long distance for O(2): Mg(2)−O(2)=2.66 Å. It is worth noting that all the cationic sites in Mg2La3Sb3O14 are centrosymmetric, unlike Ca2La3Sb3O14, in which because of the cationic disorder the “center of inversion” is just a statistical average and is not retained at a local level. The disordered structure of rhombohedral pyrochlore Ca2La3Sb3O14 can be also modeled by constructing one hexagonal unit cell containing three formula units based on the description of the ordered rhombohedral primitive cell Ca(A)Ca(B)La3(A′)Sb3O14 and then exchanging a pair of La3+ and Ca2+ ions between the A and A′ sites. In this structural model, the occupancies of La3+ ions are 33.3% and 0 on the A and B sites, respectively, while the substituted amount of Ca2+ ions on the A′ site is 11.1%. Such site occupations are D


Article

Inorganic Chemistry Table 3. Refined (at Room Temperature) and Calculated Crystallographic Data of Mg2La3Sb3O14a fractional atomic coordinates x/a

y/b

z/c

element

site

refined

calculated

refined

calculated

refined

calculated

Biso (Å)

Mg(1) Mg(2) Sb La O(1) O(2) O(3)

3a (B) 3b (A) 9d (B′) 9e (A′) 6c 18h 18h

0 0 0.5 0.5 0 0.5360(5) 0.1394(5)

0 0 0.5 0.5 0 0.53214 0.14278

0 0 0 0 0 0.4640(5) 0.8606(5)

0 0 0 0 0 0.46786 0.85722

0 0.5 0.5 0 0.3883(6) 0.1473(4) 0.9419(4)

0 0.5 0.5 0 0.38807 0.14539 0.94235

0.010(5) 0.021(4) 0.008(3) 0.012(4) 0.011(3) 0.032(5) 0.025(4)

a Refined and calculated lattice parameters (Å): for a and b (a = b), 7.5092(1) and 7.47326, respectively; for c, 17.7193(1) and 17.64448, respectively.

collected into Tables 5 and S4, respectively. Inspection of Table 5 shows that the structure in which the antisite defects

Table 4. Selected Bond Distances (Å) in Mg2La3Sb3O14 bond

refined

Mg(1)−O(3) Mg(2)−O(1) Mg(2)−O(2) Sb−O(2) Sb−O(3) La−O(1) La−O(2) La−O(3)

2.085(6) × 6 1.98(1) × 2 2.658(7) × 6 2.005(3) × 4 1.956(7) × 2 2.376(5) × 2 2.651(7) × 2 2.579(5) × 4

calculated 2.1096 1.9749 2.6006 2.0176 1.9482 2.3637 2.5989 2.5400

× × × × × × × ×

6 2 6 4 2 2 2 4

Table 5. Calculated Total Energies Per Formula Unit (in eV) of the Nonequivalent Structural Configurations Contributing to the Description of the Disordered Structure for One Unit Cell of Ca2La3Sb3O14a space group R3̅m (hexagonal cell; Z = 3)

very close to the experimentally refined results given by Fu and IJdo.5 From the point of view of point defect theory, the Ca− La exchange between A and A′ sites can be regarded as the generation of a pair of the antisite defects LaCa(A) and CaLa(A′) with respect to the ordered crystal structure, and the stability of such a “defective” system always depends on the distance between LaCa(A) and CaLa(A′), as revealed by the behaviors of the antisite defects YAl and AlY in yttrium aluminum garnet.25 On the basis of classification of the distance before relaxation between the A and A′ sites in one hexagonal unit cell of rhombohedral pyrochlore Ca2La3Sb3O14, two different structural configurations for a pair of the antisite defects LaCa(A) and CaLa(A′) can be created and are shown in Figure 3. The geometry optimization was first performed for the two structural configurations, and then the related total energies and optimized structural data were obtained and

I2/m (monoclinic cell; Z = 2)

structural configuration Ca2(A)La(A)Ca3(B)La8(A′) Ca(A′)Sb9O42 d(LaCa(A)−CaLa(A′)) = 3.7699 Åc d(LaCa(A)−CaLa(A′)) = 6.5503 Åc Ca2(2a)Ca(2c)La(2c)La4(4f) Ca(2b)La(2b)Sb6O28 d(LaCa(2c)−CaLa(2b)) = 3.8457 Åc d(LaCa(2c)−CaLa(2b)) = 6.6986 Åc

Etot(Ca(A)Ca(B) La3(A′)Sb3O14)b

0.2069 0.3213

0.2079 0.3365

a

Both the rhombohedral pyrochlore-type and monoclinic weberitetype structures are considered. bThe calculated total energy of one fully ordered rhombohedral primitive cell of Ca2La3Sb3O14 is set as zero as a reference. cThe structural data are taken from the experimental refinement results.2,5

LaCa(A) and CaLa(A′) are as close as possible is the most stable. Therefore, we can conclude that the disorder in

Figure 3. Schematic representations of two different structural configurations for a pair of the antisite defects LaCa (A) and CaLa (A′) in one hexagonal unit cell of the rhombohedral pyrochlore Ca2La3Sb3O14, classified according to the A−A′ distance before relaxation: (a) 3.7699 Å and (b) 6.5503 Å. E


Article

Inorganic Chemistry

Figure 4. Schematic representations of the experimentally refined (a−c) and calculated (d−f) local coordination environments of the three cationic sites A, A′, and B in rhombohedral pyrochlore Ca2La3Sb3O14. Here all the central sites are assumed to be occupied by Eu3+ ions. The arrows in panels d−f are used to indicate which cationic positions in the outer coordination sphere of Eu3+ ions result in the breaking of the local inversion symmetry.

Figure 5. (a) Crystal structure of monoclinic weberite Ca2La3Sb3O14. (b) Schematic representation of the most stable structural configuration for one unit cell of disordered monoclinic weberite Ca2La3Sb3O14. The second nonequivalent structural configuration can be obtained by exchanging the La3+ and Ca2+ ions on two 2b positions.

the most stable configuration reported in Table 5 is depicted in Figure 4, together with the experimentally refined ones adopting the virtual or “mixed” cationic species “Ca/La” to keep the “center of inversion”. Such intuitive schematic representations straightforwardly support our statement given in the Introduction. More explanations about breaking of the local inversion symmetry are given in the spectroscopic analysis section. In addition, from a thermodynamic point of view, the presence of the Ca−La antisite disorder in

Ca2La3Sb3O14 is mainly due to Ca−La exchange between the A and A′ sites located as close as possible. The symmetry analysis of the two optimized structural configurations indicates that the inversion symmetry is totally lost for all ionic sites, and this is due to the occupation of different cations (Ca2+ and La3+) in the same crystal site with multiplicity higher than 1 (i.e., A or A′ sites with multiplicity of 3 and 9, respectively, when hexagonal unit cell is considered). The geometry of the coordination environments of the three sites A, A′, and B in F


Article

Inorganic Chemistry

structural model can describe as well the structure of Ca2La3Sb3O14, since any rhombohedral cell may have a derivative monoclinic subcell.5 Nevertheless, if the principle of minimum energy as a fundamental concept used in physics and chemistry is obeyed, then the crystal phase R3̅m is recommended as the best choice for understanding the structure of Ca2La3Sb3O14. Luminescence of 1% Eu3+ Doped Mg2La3Sb3O14 and 1% Eu 3+ Doped Ca 2 La 3 Sb 3 O 14 . Room temperature luminescence emission spectra of Eu3+ doped Mg2La3Sb3O14 and Ca2La3Sb3O14 samples are shown in Figure 6. The samples

Ca2La3Sb3O14 can be tested by calculating the energy cost of the generation of a pair of the antisite defects LaCa(A) and CaLa(A′) in the structure of Ca(A)Ca(B)La3(A′)Sb3O14 and comparing with the thermal energy available at 300 K (kT = 0.025 eV). Inspection of Tables 2 and 5 shows that the supercell size has a remarkable influence on the determination of the formation energy of such a pair of antisite defects. Therefore, the defect formation energy calculation had to be implemented in a much bigger supercell containing 8 formula units to eliminate the finite-size effect, and the calculated value was found to be 0.09 eV, which is comparable to 0.025 eV. For the sake of comparison, the Mn−La exchange energy cost between the A and A′ sites in Mn2La3Sb3O14 was also evaluated, and its value converged to 0.4 eV with increasing the supercell size from 1 to 32 formula units. The calculation results presented here for Ca- and Mn-based compounds suggest that the presence of the antisite disorder is not possible in Mn2La3Sb3O14 but can be expected in Ca2La3Sb3O14, at least at 300 K. Finally, let us comment on the choice of the space group of Ca2La3Sb3O14. The earlier structural model of Ca2La3Sb3O14 was the one of a weberite described in the monoclinic space group I2/m, as shown in Figure 5a. The Wyckoff sites 2a, 4f, 4e, and 2d are fully occupied by Ca, La, Sb, and Sb, respectively, and therefore they are strictly ordered, whereas the 2c and 2b sites are shared by Ca2+ and La3+ ions in the ratio of 1:1. The description of the oxygen coordination environments of each cationic site in the structure is not discussed in detail because the reader can acquire the knowledge by inspecting Figure 5a or referring to ref 4. Analogously to what has been done for the rhombohedral space group before, the disorder between Ca2+ and La3+ ions on the 2c and 2b sites was first considered in the triclinic primitive cell with one chemical formula and thus two symmetry-independent structural configurations Ca(2a)Ca(2c)La2(4f)La(2b)Sb3O14 and Ca(2a)La(2c)La2(4f)Ca(2b)Sb3O14 were found and then relaxed for the total energy comparison (the optimized structural data can be found in Table S5). The results of the total energy calculations on the two configurations after relaxation suggest that the former is more stable and the latter is 0.4869 eV higher than the former. Therefore, the disordered weberite structure of Ca2La3Sb3O14 can be modeled starting from the former monoclinic unit cell with two formula units, that is, Ca2(2a)Ca2(2c)La4(4f)La2(2b)Sb6O28, upon exchanging a pair of Ca and La cations between 2c and 2b sites. That gives rise to two nonequivalent configurations for a pair of the so-called antisite defects LaCa(2c) and CaLa(2b) appearing, both described by the formula Ca2(2a)Ca(2c)La(2c)La4(4f)Ca(2b)La(2b)Sb6O28 but having two different LaCa(2c)−CaLa(2b) distances (see Table 5 and Figure 5b). Total energy calculations with the structure optimization were performed for them, and the obtained total energy values and optimized structural data are given in Tables 5 and S6, respectively. From the inspection of Table 5, it is evident that, similarly to the case of the rhombohedral space group, the disorder in the monoclinic description of Ca2La3Sb3O14 is mainly due to the Ca−La exchange between two sites (2c and 2b) located as close as possible. The calculated total energy per formula unit of the most stable structural configuration after the Ca−La exchange is only 0.001 eV higher than the one calculated in the case of the rhombohedral pyrochlore structure (see Table 5). This quantitatively explains why the monoclinic weberite-type

Figure 6. Room temperature luminescence emission spectra of Mg2La3Sb3O14 (blue line) and Ca2La3Sb3O14 (orange line) both doped with 1% of Eu3+.

were efficiently excited at the most intense Eu3+ excitation peak [for the Ca2La3Sb3O14 host around 392 nm (7F0 → 5L6 transition); for Mg2La3Sb3O14 around 524 nm (7F0 → 5D1), Figure S1]. It is not surprising that in the Mg-analogue the magnetic dipole 7F0 → 5D1 transition dominates the excitation spectrum. In fact, in this host, the luminescent Eu3+ ion is expected to substitute for La3+ in a centrosymmetric site (C2h point symmetry), and on the basis of the selection rules, only the transitions with a magnetic dipole character are allowed. Therefore, the three dominant peaks in the emission spectrum of Eu3+ doped Mg2La3Sb3O14 can be attributed to the three Stark levels of the 5D0 → 7F1 magnetic dipole transition in agreement with a C2h site symmetry (Figure 6). The low intensity of emission peak around 575 nm and the ones above 600 nm may belong to a small amount of an impurity that is detected by XRD upon collecting the pattern with very long time of exposure (data not shown). As the intensity of these emission peaks is dependent on the excitation wavelength (Figure S2), this evidence could be explained assuming that a selective excitation of Eu 3+ ion within this impurity (presumably the monoclinic La3SbO7, Cmcm space group26) can be obtained upon a range of excitation wavelengths. As observed by Srivastava et al.7 the Eu3+ luminescence suggests that the point (site) symmetry of the La3+ cations in Ca2La3Sb3O14 deviates from the one expected from crystallography. To this day, the two space groups (I2/m space group or its revision (R3̅m))2,5 would predict a centrosymmetric location for all cations although the R3̅m is preferred by the present investigation. In contrast, the emission spectrum of Ca2La3Sb3O14:Eu3+ is dominated by 5D0 → 7F2 and 5D0 → 7F0 transitions (Figure 6, upper trace), which is compatible with Eu3+ ions located in a noncentrosymmetric site. G


Article

Inorganic Chemistry

respectively, can be bridged by less than four Sb−O phonons. This implies that fast multiphonon relaxation processes31 will quench the emissions from the 5D1 and 5D2 excited states. Figure 7 exhibits the luminescence decay curves of Eu3+ 5D0 level in Ca2La3Sb3O14 and Mg2La3Sb3O14. In the Ca material,

This behavior seems to be paradoxical but could be reasonably explained by considering the fact that the XRD technique describes the averaged structure whereas the local symmetry may deviate from the apparent one. In fact, the outer coordination spheres of La3+ (and of the spectral probe Eu3+) in Ca2La3Sb3O14 are affected by the Ca−La cationic disorder, and consequently the local symmetry of the metal ions is actually no longer strictly centrosymmetric. This effect of the outer coordination spheres on the local symmetry at the Eu3+ dopant is similar to the case reported many years ago in a seminal paper by Blasse and Bril27 about the luminescence spectroscopy of Eu3+ in NaLuO2 and NaGdO2. Although both compounds have the rock salt crystal structure, they exhibit different ordering of the Na and Ln (Ln = Lu and Gd) cations. Because of the different ordering, the Eu3+ occupies an inversion center in NaLuO2 (D3d symmetry) but not in NaGdO2 (D2d symmetry). Consequently, the luminescence spectra of Eu3+ are very different in the two structures: in NaLuO2:Eu3+ only the 5D0 → 7F1 magnetic dipole transitions are observed (the only ones allowed for Eu3+ in a crystal site with inversion symmetry), whereas the 5D0 → 7F2 forced electric dipole transitions dominate the emission spectrum in NaGdO2:Eu3+. When one considers the comparison between the present case and the one reported by Blasse, it is important to clarify the concept. In both cases, the loss of the local inversion symmetry is due to an effect of the outer coordination sphere of the metal ion, but, in the case of rock-salt-type double oxides, whereas the local and “averaged” geometric distributions of the ionic species around the metal ion are the same, a change of the point symmetry (from D3d to D2d) occurs. In contrast, the loss of the inversion symmetry in Ca2La3Sb3O14 is a more sophisticated mechanism, as it must be considered strictly a local phenomenon, since no change of the average point symmetry (C2h) of the trivalent cation is observed, passing from Mg2La3Sb3O14 to Ca2La3Sb3O14. The graphical evidence is given in Figure 4, where the crystallographic and calculated coordination environments around Eu3+ ions are compared. In general, it is difficult to predict the distance beyond which the cationic disorder does not affect the Eu3+ hypersensitive transition. In the present case, the second coordination sphere made of La/M cations and affecting the Eu3+ electric dipole transition is at a distance of 3.6−3.8 Å from the luminescent ion. This is fully compatible with the similar outer coordination sphere effect observed for NaGdO 2 and NaLuO2 and discussed above. In the case of NaGdO2, the distances between Eu3+ dopant ion and the metal cations in the second coordination sphere (responsible of the removal of the inversion symmetry) are in the 3.30−3.52 Å range. In addition, it is useful to repeat that the La3+ (and Ca2+) ions occupy three different crystal sites in Ca2La3Sb3O14. Inspection of the emission spectrum (Figure 6, upper trace) clearly shows the multisite nature of the Eu3+ emission. Due to the one-to-one correspondence between the number of emitting sites and the number of the 5D0 → 7F0 components,28 we can propose the presence of at least three different sites for the Eu3+ dopant, in accordance with the crystal structure. It should be also pointed out that the radiative emissions from the Eu3+ 5D1 and 5D2 excited states are not expected in this structure for the following reasons. The lattice maximum phonon energy is around 742 cm−1.29 Therefore, the 5D1−5D0 and the 5D2−5D1 energy gaps30 of about 1730 and 2460 cm−1,

Figure 7. Luminescence decay curves of the 5D0 excited state in Eu3+ doped Ca2La3Sb3O14 (excitation into the 5L6 state; λem = 580 nm) and Mg2La3Sb3O14 (λex = 524 nm, λem = 587 nm).

the decay curve is not exactly single exponential, and the observed decay constant is around 2.5 ms (1/e folding time) (Figure 7), independent of the emission wavelength. In the Mg compound, the luminescence lifetime of the 5D0 excited state is around 4.0 ms (1/e folding time) (Figure 7). Also in this case, the observed decay constant is independent of the observed emission wavelength (λem = 583, 587, and 599 nm, Figure S3). The difference of the observed lifetimes in the two hosts is clearly due to a faster radiative decay in the case of the Ca-based analogue, due to the activation of forced electric dipole transitions in the local noncentrosymmetric environment.

■

CONCLUSIONS In conclusion, this paper has probed the crystal structure of M2La3Sb3O14 (M = Mg or Ca). In particular, DFT calculations confirm what has been observed by crystallography: (i) R3̅m is the more reasonable space group for the two hosts; (ii) while Mg2La3Sb3O14 shows an ordered cationic configuration, Ca2La3Sb3O14 presents a disordered distribution of Ca2+ and La3+ cations. The latter condition has a dramatic effect on the Eu3+ luminescence in the two host materials. In fact, even if the Eu3+ ions formally occupy centrosymmetric sites in both matrices, in the case of Ca2La3Sb3O14, the presence of disorder in the outer coordination spheres removes the local inversion symmetry, therefore enabling forced electric dipole transitions. Such discovery can be conveniently exploited for the design of new phosphor materials in which the cationic disorder can induce the breaking of the local inversion symmetry of the luminescent lanthanide ion to increase the probability and the efficiency of the radiative transition. Finally, it may be useful to remark that this study persuasively shows that a thorough understanding of the symmetry features of the local atomic structure in complex materials requires the synergic use of various experimental techniques and computational methods, which provide information on different length scales. Also XANES spectroscopy is able to provide details regarding the local structure around metal ions and in particular lanthanide ions.32 The use of this technique, which is beyond the scope of H


Article

Inorganic Chemistry

and Pr3+ in the Weberite, Ca2La3Sb3O14. Opt. Mater. 2014, 38, 248− 251. (8) Sanders, M. B.; Baroudi, K. M.; Krizan, J. W.; Mukadam, O. A.; Cava, R. J. Synthesis, Crystal Structure, and Magnetic Properties of Novel 2D Kagome Materials RE3Sb3Mg2O14 (RE = La, Pr, Sm, Eu, Tb, Ho): Comparison to RE3Sb3Zn2O14 Family. Phys. Status Solidi B 2016, 253, 2056−2065. (9) Yu, C.-J.; Emmerich, H. An Efficient Virtual Crystal Approximation that can be Used to Treat Heterovalent Atoms, Applied to (1−x)BiScO3−xPbTiO3. J. Phys.: Condens. Matter 2007, 19, No. 306203. (10) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos National Laboratory Report LAUR 86748: Los Alamos, NM, 2004. (11) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M.; Causà, M.; Noël, Y. CRYSTAL14 User’s Manual; University of Torino: Torino, Italy, 2014. (12) Demichelis, R.; Civalleri, B.; Ferrabone, M.; Dovesi, R. On the Performance of Eleven DFT Functionals in the Description of the Vibrational Properties of Aluminosilicates. Int. J. Quantum Chem. 2010, 110, 406−415. (13) Evarestov, R. A. Quantum Chemistry of Solids: The LCAO First Principles Treatment of Crystals; Spring-Verlag: Heidelberg, 2007. (14) Towler, M. D.; Allan, N. L.; Harrison, N. M.; Saunders, V. R.; Mackrodt, W. C.; Apra, E. Ab Initio Study of MnO and NiO. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 5041−5054. (15) Valenzano, L.; Torres, F. J.; Doll, K.; Pascale, F.; Zicovichwilson, C. M.; Dovesi, R. Ab Initio Study of the Vibrational Spectrum and Related Properties of Crystalline Compounds; the Case of CaCO3 Calcite. Z. Phys. Chem. 2006, 220, 893−912. (16) Valenzano, L.; Noel, Y.; Orlando, R.; Zicovich-wilson, C. M.; Ferrero, M.; Dovesi, R. Ab Initio Vibrational Spectra and Dielectric Properties of Carbonates: Magnesite, Calcite and Dolomite. Theor. Chem. Acc. 2007, 117, 991−1000. (17) Bredow, T.; Jug, K.; Evarestov, R. A. Electronic and Magnetic Structure of ScMnO3. Phys. Status Solidi B 2006, 243, R10−R12. (18) Metz, B.; Stoll, H.; Dolg, M. Small-Core MulticonfigurationDirac−Hartree−Fock-Adjusted Pseudopotentials for Post-d Main Group Elements: Application to PbH and PbO. J. Chem. Phys. 2000, 113, 2563−2569. (19) Cao, X.; Dolg, M. Segmented Contraction Scheme for SmallCore Lanthanide Pseudopotential Basis Sets. J. Mol. Struct.: THEOCHEM 2002, 581, 139−147. (20) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (21) Fu, W. T.; IJdo, D. J. W. Crystal Structure of Mn2Ln3Sb3O14 (Ln = La, Pr and Nd): A New Ordered Rhombohedral Pyrochlore. J. Solid State Chem. 2014, 213, 165−168. (22) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (23) Li, K.; Hu, Y. F.; Wang, Y. X.; Kamiyama, T.; Wang, B. W.; Li, Z. F.; Lin, J. H. Syntheses and Properties of a Family of New Compounds RE3Sb3Co2O14 (RE = La, Pr, Nd, Sm−Ho) with an Ordered Pyrochlore Structure. J. Solid State Chem. 2014, 217, 80−86. (24) Sanders, M. B.; Krizan, J. W.; Cava, R. J. RE3Sb3Zn2O14 (RE = La, Pr, Nd, Sm, Eu, Gd): A New Family of Pyrochlore Derivatives with Rare Earth Ions on a 2D Kagome Lattice. J. Mater. Chem. C 2016, 4, 541−550. (25) Munoz-Garcia, A. B.; Artacho, E.; Seijo, L. Atomistic and Electronic Structure of Antisite Defects in Yttrium Aluminum Garnet: Density-Functional Study. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, No. 014105. (26) Siqueira, K. P. F.; Borges, R. M.; Granado, E.; Malard, L. M.; de Paula, A. M.; Moreira, R. L.; Bittar, E. M.; Dias, A. Crystal Structure of Fluorite-Related Ln3SbO7 (Ln = La−Dy) Ceramics Studied by

the present investigation, is being planned in order to confirm the conclusions drawn in the present contribution.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01261. Luminescence excitation and emission data, excited state decay kinetics data, and tables of DFT calculation geometries (PDF) Accession Codes

CCDC 1843048 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail address: [email protected]. *E-mail address: [email protected]. ORCID

Fabio Piccinelli: 0000-0003-0349-1960 Irene Carrasco: 0000-0002-7854-4214 Marco Bettinelli: 0000-0002-1271-4241 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully thank Erica Viviani (Univ. Verona) for expert technical assistance. All the calculations were performed on the high-performance computational platform managed by Prof. Shuai Yuan working in Chongqing University of Posts and Telecommunications (CQUPT). C.-G. Ma acknowledges financial support from China Scholarship Council (CSC File No. 201607845015), Scientific Research Foundation for Returned Overseas Chinese Scholars offered by Chinese Ministry of Human Resources and Social Security (Grant No. [2014] 167), and Wenfeng High-end Talents Project of CQUPT (Grant No. W2016-01).

■

REFERENCES

(1) Burchard, G.; Rü dorff, W. Polynäre Oxide A5B3O14 mit Chiolithstruktur. Z. Anorg. Allg. Chem. 1978, 445, 79−90. (2) Au, Y. S.; Fu, W. T.; IJdo, D. J. W. Crystal Structure of Ca2Ln3Sb3O14 (Ln = La, Pr, Nd and Y): A Novel Variant of Weberite. J. Solid State Chem. 2007, 180, 3166−3171. (3) Groen, W. A.; IJdo, D. J. W. Distrontium Diantimonate(V). A Rietveld Refinement of Neutron Powder Diffraction Data. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1988, 44, 782−784. (4) Brik, M. G.; Srivastava, A. M.; Bettinelli, M. Ab Initio Calculations of the Structural and Electronic Properties of Ca2La3Sb3O14 Weberite at Ambient and Elevated Hydrostatic Pressure. ECS J. Solid State Sci. Technol. 2014, 3, R1−R4. (5) Fu, W. T.; Ijdo, D. J. W. New Rhombohedral Pyrochlores Ca2Ln3Sb3O14 (Ln = Lanthanide and Y): Space Group Revised. J. Solid State Chem. 2015, 229, 330−335. (6) Mighell, A. D. Lattice Metric Singularities and Their Impact on the Indexing of Powder Patterns. Powder Diffr. 2000, 15, 82−85. (7) Srivastava, A. M.; Camardello, S. J.; Comanzo, H. A.; Beers, W. W.; Brik, M. G.; Trevisani, M.; Bettinelli, M. Luminescence of Eu3+ I


Article

Inorganic Chemistry Synchrotron X-ray Diffraction and Raman Scattering. J. Solid State Chem. 2013, 203, 326−332. (27) Blasse, G.; Bril, A. On the Eu3+ Fluorescence in Mixed Metal Oxides. V. the Eu3+ Fluorescence in the Rocksalt Lattice. J. Chem. Phys. 1966, 45, 3327−3332. (28) Binnemans, K. Interpretation of Europium (III) Spectra. Coord. Chem. Rev. 2015, 295, 1−45. (29) Haeuseler, H. Infrared and Raman Spectra and Normal Coordinate Calculations on Trirutile-Type Compounds. Spectrochim. Acta, Part A 1981, 37, 487−495. (30) Carnall, W. T.; Crosswhite, H.; Crosswhite, H. M. Energy Level Structure and Transition Probabilities in the Spectra of the Trivalent Lanthanides in LaF3; Argonne National Laboratory Report ANL 78XX-95: Lemont, IL, 1978. (31) Van Dijk, J. M. F.; Schuurmans, M. F. H. On the Nonradiative and Radiative Decay Rates and a Modified Exponential Energy Gap Law for 4f−4f Transitions in Rare-Earth Ions. J. Chem. Phys. 1983, 78, 5317−5323. (32) Asakura, H.; Shishido, T.; Teramura, K.; Tanaka, T. Local Structure and La L1 and L3-Edge XANES Spectra of Lanthanum Complex Oxides. Inorg. Chem. 2014, 53, 6048−6053.

J


Disorder-Induced Breaking of the Local Inversion Symmetry in

Recommend Documents