Growth, single-crystal structure, and magnetic properties of the double molybdate KYb(MoO4)2

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Highlights

  • Flux-growth of millimeter size, colorless, and transparent KYb(MoO4)2 single crystals.

  • KYb(MoO4)2 crystallizes in the orthorhombic symmetry, space group Pbcn (No. 60).

  • Ordered distribution of the K+ and Yb3+ cations on C2 symmetry positions.

  • Existence of “Mo2O8 dimers” confirmed by the Raman band at 736 cm−1.

  • Large Weiss constant value of θp = −52 K.

Abstract

Crystals of the rare earth double molybdate KYb(MoO4)2 were grown by spontaneous nucleation from a K2Mo4O13 flux. The typical crystals were colorless and elongated, well-developed plates. The material has been characterized by powder X-ray diffraction (PXRD), energy-dispersive X-ray spectrometry (EDS), and Raman spectroscopy methods. The structural properties have been investigated through single-crystal X-ray diffraction at room temperature. KYb(MoO4)2 crystallizes in the orthorhombic symmetry, space group Pbcn (No. 60), a = 5.0395(2), b = 18.2821(9), c = 7.8635(4) Å, and Z = 4. A complete ordering of the Yb3+ and Mo6+ cations has been found. The Yb3+ ions are located in distorted YbO8 antiprisms which form chains of edge-sharing, aligned along the c-axis while the Mo6+ ions are placed in a slightly deformed tetrahedral environment of oxygen atoms. Double bridge modes of the “Mo2O8 dimers” have been identified in the unpolarized Raman spectra. The temperature-dependent magnetic susceptibility of KYb(MoO4)2 follows a Curie–Weiss law with a Weiss constant of −52 K.

Introduction

Double molybdates and tungstates (DMo/DW) of general formula AT(XO4)2 with X = MoVI or WVI, a monovalent element A and a trivalent or lanthanide element T, exist with a wide variety of polymorphic forms, mainly tetragonal and orthorhombic, which largely depend on the ratio of the ionic radii A+/T3+ [[1], [2], [3], [4], [5], [6]]. This structural diversity results in several properties such as negative thermal expansion, third-harmonic generation or ionic conduction, and applications in modern technology [[7], [8], [9], [10], [11]]. Special attention has been paid to trigonal DMo and DW due to their simple structure and their ability to undergo ferroelastic phase transitions over wide temperature ranges [2,11]. Some DMo and DW phases, such as orthorhombic (Pbna) KYMo2O8, have potential applications as microchip lasers due to their perfect cleavage in the (100) planes [2,8,10].

The compounds AT(XO4)2 have generated some interest as new materials for laser systems because the isostructural substitution of the trivalent element T by optically active lanthanides Ln is possible over a wide concentration range, sometimes up to the stoichiometric composition. For example, KGd(WO4)2 is well-known as an efficient Raman material that contains an ionic molecular complex that exhibits active oscillations of Stimulated Raman Scattering [12].

Moreover, the lanthanide-based, magnetically concentrated crystals of double tungstates and molybdates are shown to display optical properties such as photon upconversion emission as well as interesting magnetic properties [13,14]. Single-molecule magnets (SMM) and single-ion magnets (SIM) systems based on oxygen coordination polyhedra around Nd3+ and/or Yb3+ lanthanides have been synthesized [15,16].

The present work is part of a study aimed at the synthesis and structural evaluation of the Ln-based magnetically-concentrated double molybdates, KLn(MoO4)2. The formation of several molybdates of this family has already been reported in the past [[1], [2], [3],8,10,15,[17], [18], [19], [20], [21]] but very fragmentary information is available for KYb(MoO4)2 [14,19,21].

We present here the flux-growth of the orthorhombic phase KYb(MoO4)2 and its single-crystal structure determined at room temperature. The magnetization and hysteresis loops have been measured in a temperature range from 1.8 to 300 K to study its magnetic behavior.

Section snippets

Material preparation

The starting reagents were commercial Alfa Aesar products MoO3 (99.95%), K2CO3 (99%), and Yb2O3 (99.9%), and were used as received. Crystals were obtained using the high-temperature flux growth technique through spontaneous crystallization. The K2Mo4O13 flux was previously synthesized via a solid-state reaction at 500 °C for two weeks, from powders of K2CO3 alkali carbonate (31 g) and MoO3 molybdenum oxide (129 g). A total amount of 20 g of an intimate mixture of Yb2O3 and K2Mo4O13 in a weight

Flux and crystals characterizations

The formation of the K2Mo4O13 flux in the powder form was characterized by room-temperature X-ray powder diffraction, Fig. 1. It was positively compared with the Powder Diffraction Standards Card N° 00-27-0416 [27].

Spontaneous nucleation yielded sub-millimeter and millimeter-sized plate-shaped crystals, and no post-growth annealing was applied. Fig. 2 shows selected as-grown crystals with dimensions up to 6.0 × 1.2 × 0.5 mm. The transparent and colorless crystals display good optical quality,

Conclusion

KYb(MoO4)2 double molybdate was obtained for the first time as transparent and colorless single crystals, using a high-temperature solution technique. The flux-grown material crystallizes in the orthorhombic symmetry with the MoVI valence state and displays an ordered distribution of the K+ and Yb3+ cations, both on C2 symmetry positions. The structural type KY(MoO4)2 was assigned based on single-crystal structure determination. The existence of “Mo2O8 dimers” was confirmed by the presence of

Funding source

Funding for this research was provided by the Ministry of Higher Education, Scientific Research and Innovation in France, and the French National Center of Scientific Research (CNRS).

CRediT authorship contribution statement

P. Armand: Conceptualization, Data curation, Formal analysis, Writing – original draft, Writing – review & editing, Conception and design of study, Acquisition of data, Analysis and/or interpretation of data, Drafting the manuscript, Approval of the version of the manuscript. C. Reibel: Data curation, Writing – review & editing, Acquisition of data, Approval of the version of the manuscript. D. Granier: Data curation, Writing – review & editing, Acquisition of data, Approval of the version of

Declaration of competing interest

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.

Acknowledgments

Raman, magnetic, and X-ray diffraction experiments were done at the Platform of Analysis and Characterization of the Pôle Chimie Balard in Montpellier, France.

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