Structure and Properties of Ln2MoO6 Oxymolybdates (Ln = La, Pr, Nd) Doped with Magnesium

Ekaterina Orlova,Timofei Sorokin,Nataliya Sorokina,Elena Kharitonova,Valentina Voronkova,Alexander Antipin,Nataliya Novikova

doi:10.3390/cryst11060611

Abstract

The literature data and the results obtained by the authors on the study of the structure and properties of a series of polycrystalline and single-crystal samples of pure and Mg-doped oxymolybdates Ln2MoO6 (Ln = La, Pr, Nd) are analyzed. Presumably, the high-temperature phase I41/acd of Nd2MoO6 single crystals is retained at room temperature. The reason for the loss of the center of symmetry in the structures of La2MoO6 and Pr2MoO6 and the transition to the space group I4¯c2 is the displacement of oxygen atoms along the twofold diagonal axes. In all structures, Mg cations are localized near the positions of the Mo atoms, and the splitting of the positions of the atoms of rare-earth elements is found. Thermogravimetric studies, as well as infrared spectroscopy data for hydrated samples of Ln2MoO6 (Ln = La, Pr, Nd), pure and with an impurity of Mg, confirm their hygroscopic properties.

Highlights

During decades, tungstates, molybdates, and binary systems of the Ln2 O3 –Mo(W)O3 type, especially in the concentration range 25–50 mol% of Ln2 O3 oxide, have been intensively studied [1,2,3,4,5,6]
The presented work is a brief review in which we summarize, analyze and compare the structural features and properties of Mg-doped Ln2 MoO6 oxymolybdates with large rare earth cations (Ln = La, Pr, Nd), previously published and supplemented by studies of a number of new properties using powder X-ray diffraction, thermogravimetry, and infrared spectroscopy
The minimum deviations of the coordinates of oxygen atoms in space group I4c2 from their positions in non-polar space group I41 /acd are observed for the Nd2 MoO6 structure in comparison with similar values for the La2 MoO6 and Pr2 MoO6 structures (Table 2), which indicates that the high-temperature phase I41 /acd of the Nd2 MoO6 single crystal is presumably fixed at room temperature

Summary

Introduction

Tungstates, molybdates, and binary systems of the Ln2 O3 –Mo(W)O3 type, especially in the concentration range 25–50 mol% of Ln2 O3 oxide, have been intensively studied [1,2,3,4,5,6]. Scheelite-like 1:3 phases (the 1:3 ratio of molar concentrations of oxides, i.e., the 1:3 composition) with excellent luminescent properties are distinguished among them. Oxymolybdates and oxytungstates are chemically stable and have high refractive indices. These compounds doped with Eu exhibit intense luminescence [10,11] and are used as microwave ceramics [12,13]. Other properties of these compounds are poorly understood. Let us focus our attention on the study of a number of new properties of the rather well-studied phases of oxymolybdates and oxytungstates

Methods

Results

Conclusion