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Crystal structure refinement of new vanadates Ca8−x Pbx CdBi(VO4)7

Published online by Cambridge University Press:  29 March 2017

Daria Petrova ,
Dina Deyneko ,
Sergey Stefanovich  and
Bogdan Lazoryak
Daria Petrova*
Affiliation:
Chemistry Department, Lomonosov Moscow State University, Leninskie gory, d. 1, Moscow, Russia Physical and Colloid Chemistry Department, Gubkin Russian State University of Oil and Gas (National Research University), Leninskiy prospekt, d. 65, Moscow, Russia
Dina Deyneko
Affiliation:
Chemistry Department, Lomonosov Moscow State University, Leninskie gory, d. 1, Moscow, Russia Shubnikov Institute of Crystallography RAS, Leninskiy prospekt, d. 59, Moscow, Russia
Sergey Stefanovich
Affiliation:
Chemistry Department, Lomonosov Moscow State University, Leninskie gory, d. 1, Moscow, Russia L.Ya. Karpov Institute of Physical Chemistry, Obukhova per., d. 3, Moscow, Russia
Bogdan Lazoryak
Affiliation:
Chemistry Department, Lomonosov Moscow State University, Leninskie gory, d. 1, Moscow, Russia
*
a) Author to whom correspondence should be addressed. Electronic mail: petrova.msu@gmail.com

Abstract

New Ca8−x Pbx CdBi(VO4)7 with the whitlockite-type structure were prepared by a standard solid-state method in air. Le Bail decomposition was used to determine unit-cell parameters. Structural refining was carried out by Rietveld's method. It is found that Bi3+ cations located partially in M1 and M2 sites along with calcium, while M3 site is settled in half by Pb2+-ions. Second-harmonic generation demonstrate highest non-linear optical activity and along with dielectric investigations indicate polar space group R3c.

Keywords

crystal structurewhitlockitenon-linear optical activityphase transitionRietveld refinement

Information

Type
Technical Articles
Information
Powder Diffraction , Volume 32 , Supplement S1 , September 2017 , pp. S106 - S109
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Copyright
Copyright © International Centre for Diffraction Data 2017 

I. INTRODUCTION

Crystal chemistry tuning of dielectric and optical properties in whitlockite-like phosphate and vanadate systems makes them attractive for scientists (Liang et al., Reference Liang, Yuan, Yang and Chen2010; Wu et al., Reference Wu, Huang and Seo2011; Zhang et al., Reference Zhang, Wang, Guo, Zhang, Wen, Liu and Huang2011) because of isovalent and heterovalent substitutions (Lazoryak, Reference Lazoryak1996). Cations location in the host lattice has a direct impact on many properties of whitlockite-like compounds. The whitlockite-like vanadates crystallize in structure with five cation sites M1, M2, M3, M4, and M5 (Yashima et al., Reference Yashima, Sakai, Kamiyama and Hoshikawa2003) different in size and oxygen coordination. Varying of an occupation factor of the M4 site from zero to unity allows heterovalent substitutions. The M5 site has octahedral coordination and can be occupied by a large number of elements, such as Ca2+, Cd2+, transition metals in the oxidation states of 2+ and 3+, and small-radius rare earths ions.

An advanced material with ferroelectric, ionic conductivity, and non-linear-optical properties being Ca9Bi(VO4)7 (Evans et al., Reference Evans, Huang and Sleight2001; Lazoryak et al., Reference Lazoryak, Baryshnikova, Stefanovich, Malakho, Morozov, Belik, Leonidov, Leonidova and Van Tendeloo2003). Highly polarizable asymmetric metal–oxygen bonds are formed by cations with a lone pair of electron (Bi3+, Pb2+, Tl+, Te4+). As a result increasing the non-linear optical activity of compounds usually occurs. Rise of bivalent metal concentration in Ca9−x M x Bi(VO4)7 (M 2+ = Cd2+, Zn2+) solid solutions leads to non-linear optical activities enhancement (Vorontsova et al., Reference Vorontsova, Malakho, Morozov, Stefanovich and Lazoryak2004). Additional space for high-polarizable outer Bi3+ electrons is released because of occupancy M5 site by Cd2+ cations with small radius [r VI(Cd2+) = 0.95 Å] (Shannon, Reference Shannon1976). Thereby mean range 〈M5–О〉 in Ca9Bi(VO4)7 decreases to d 〈M5−O〉 = 2.298 Å, while mean distances 〈M5–О〉 in Ca8CdBi(VO4)7 are d 〈M5–O〉 = 2.23 Å. Additionally metal-oxygen distances increase in polyhedra M1, M2, and M3. The M4 site is vacant in compounds of such type (Benarafa et al., Reference Benarafa, Kacimi, Coudurier and Ziyad2000; Evans et al., Reference Evans, Huang and Sleight2001; Vorontsova et al., Reference Vorontsova, Malakho, Morozov, Stefanovich and Lazoryak2004). Widening of spare space in the whitlockite structure is caused in greater degree by introduction of larger cations Pb2+ [r VIII(Pb2+) = 1.29 Å (Shannon, Reference Shannon1976)] as compared with Bi3+. It provides a retention rotating mobility of VO4 3− tetrahedra at lower temperatures (Deyneko et al., Reference Deyneko, Stefanovich, Mosunov, Baryshnikova and Lazoryak2013a, Reference Deyneko, Stefanovich, Mosunov, Baryshnikova and Lazoryak2013b) and, as a result, decreases the Curie points.

In this paper, structural details and an effect of crystal structure on the second-harmonic generation (SHG) efficiency and dielectric properties of Ca8−x Pb x CdBi(VO4)7 solid solutions are investigated.

II. EXPERIMENTAL

A. Synthesis of powders

Powders and ceramics of whitlockite-like structure solid solutions Ca8−x Pb x CdBi(VO4)7 at 0 ≤ x ≤ 2 were prepared by solid-state method from stoichiometric mixtures of CaCO3 (99.99%), PbO (99.8%), V2O5 (99.8%), CdO (99.8%), and Bi2O3 (99.8%). The raw materials were homogenized and reacted in Al2O3 crucibles in air at 1193 K during 150 h, and then cooled to room temperature, reground every 20 h.

B. Powder diffraction data collection and properties investigations

X-ray diffraction (XRD) patterns of powdered samples were collected at room temperature with Termo ARL powder diffractometer. The main details of the data collection are given in Table I. Lattice parameters were determined by the Le Bail decomposition (Le Bail et al., Reference Le Bail, Duroy and Fourquet1988). To refine the structure Rietveld's method was applied with using the JANA2006 software (Dusek et al., Reference Dusek, Petrícek, Wunschel, Dinnebier and Van Smaalen2001; Petrícek et al., Reference Petricek, Dusek and Palatinus2014).

Table I.

Crystallographic data and details in the data collection and refinement parameters for Ca6.5Pb1.5CdBi(VO4)7.

Optical SHG investigations were performed on graduated powders within electric furnace in one channel of optical installation, and 3-μm α-SiO2 powder used as a standard in the other channel. The both channels were identical and operated in the reflection geometry, as described in (Kurtz and Perry, Reference Kurtz and Perry1968). In each channel, the SHG signal was excited by 1.064 µm radiation of Q-switched pulsed Nd:YAG laser (Minilite-I, f = 15 Hz). Generated in the samples green light of SH at λ = 0.532 µm was registered. Measured signal from the sample under investigation was calibrated in relation to quartz standard signal from the second channel, so value Q = I 2ω /I 2ω (SiO2) always presented quantitatively SHG activity of the powder in between 293 and 1100 K.

The dielectric parameters were measured between 293 and 1073 K by a two-probe method in frequency range from 0.3 Hz to 1 MHz using Novocontrol Beta-N Impedance analyzer equipped with Probostar A cell. Full reproducibility of σ(T) curves in heating–cooling cycles indicated retention of quasi-equilibrium conditions in samples in all experiments.

III. RESULTS AND DISCUSSION

The XRD data for the systems Ca8−x Pb x CdBi(VO4)7 (0 ≤ x ≤ 2) indicated single-phase whitlockite-like structures. Figure 1 displays a portion of the observed, calculated and difference XRD powder (PXRD) pattern. PXRD pattern is similar to those of other whitlockite-like compounds. PXRD pattern did not contain any impurity reflections. Atomic coordinates and distances were refined using the pseudo-Voigt profile function by the JANA2006 program. Unit-cell parameters and some numerical characteristics illustrating the quality of the structure refinements are presented in Table I.

Figure 1.

Observed, calculated and difference PXRD patterns for Ca6.5Pb1.5CdBi(VO4)7. Tick marks denote the peak positions of possible Bragg reflections.

We used fractional coordinates of Ca8.5PbCd(PO4)7 in our initial structural model (space group R3c) (Deyneko et al., Reference Deyneko, Stefanovich, Mosunov, Baryshnikova and Lazoryak2013a, Reference Deyneko, Stefanovich, Mosunov, Baryshnikova and Lazoryak2013b). At the first stage of structure refinement Cd2+ cation located in the M5 site [6a: 0 0 0, g(Cd2+) = 1]. Successful refinement was produced through arranging Ca2+ and Pb2+ in M3, while Ca2+ and Bi3+ kept their locations in M1–M2 sites similar to other whitlockites (Lazoryak et al., Reference Lazoryak, Baryshnikova, Stefanovich, Malakho, Morozov, Belik, Leonidov, Leonidova and Van Tendeloo2003). In connection with substitution scheme (3Са2+) → (2Ln3+ + □) vacancies (□) are generated and locate in M4 site. The fractional atomic coordinates, isotropic atomic displacement parameters, cation occupancies and main relevant interatomic distances for Ca6.5Pb1.5CdBi(VO4)7 are listed in Tables II and III.

Table II.

Structural parameters for Ca6.5Pb1.5CdBi(VO4)7.

Table III.

The main interatomic distances for Ca6.5Pb1.5CdBi(VO4)7.

This ionic distribution leads to the formation of enough space for lone-pair electrons to produce non-linear optical properties. The refinement of the Ca6.5Pb1.5CdBi(VO4)7 structure leads to reasonable values of the R-factors, the interatomic distances, and the atomic displacement parameters U iso of all atoms (Table II).

SHG analysis indicated higher non-linear optical activity of Pb2+-containing compositions in comparison with Ca9Bi(VO4)7 (Evans et al., Reference Evans, Huang and Sleight2001; Lazoryak et al., Reference Lazoryak, Baryshnikova, Stefanovich, Malakho, Morozov, Belik, Leonidov, Leonidova and Van Tendeloo2003). The highest SHG signal belongs to Ca6.5Pb1.5CdBi(VO4)7. Transition from paraelectric phase to centrosymmetric group attributed to structural transformation R3c ↔ R $\overline 3 $ c. Ferroelectric phase transitions are confirmed by SHG and dielectric measurements. Curie temperature for Ca8−x Pb x CdBi(VO4)7 is between 730 and 900 K (Figure 2).

Figure 2.

Temperature dependencies dielectric constant ε for Ca8−x Pb x CdBi(VO4)7 [x = 0.5 (1), 1 kHz; 1 (2) 100 Hz, 1.5 (3) 1 kHz; 2 (4) 100 Hz].

SUPPLEMENTARY MATERIAL

The supplementary material for this article can be found at https://doi.org/10.1017/S0885715617000264

ACKNOWLEDGEMENTS

This work was supported by the Russian Foundation for Basic Research (grant numbers 14-03-01100 and 16-33-00197), and the Foundation of the President of the Russian Federation (grant number MK-7926.2016.5).

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Figure 0

Table I. Crystallographic data and details in the data collection and refinement parameters for Ca6.5Pb1.5CdBi(VO4)7.

Figure 1

Figure 1. Observed, calculated and difference PXRD patterns for Ca6.5Pb1.5CdBi(VO4)7. Tick marks denote the peak positions of possible Bragg reflections.

Figure 2

Table II. Structural parameters for Ca6.5Pb1.5CdBi(VO4)7.

Figure 3

Table III. The main interatomic distances for Ca6.5Pb1.5CdBi(VO4)7.

Figure 4

Figure 2. Temperature dependencies dielectric constant ε for Ca8−xPbxCdBi(VO4)7 [x = 0.5 (1), 1 kHz; 1 (2) 100 Hz, 1.5 (3) 1 kHz; 2 (4) 100 Hz].

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