Growth of $${\text{N}}{{{\text{d}}}_{{{\text{1}}\; - \;y}}}{\text{Eu}}_{y}^{{{\text{2}} + }}{{{\text{F}}}_{{{\text{3}}\;
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Growth of Nd1 − yEu2+ y F3 − y Single Crystals with Tysonite-Type (LaF3) Structure and Investigation of the Concentration Dependence of Some Their Properties D. N. Karimova,*, N. I. Sorokina,**, V. I. Sokolovb, and B. P. Soboleva a
Shubnikov Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia b Institute of Photon Technologies, Federal Scientific Research Centre “Crystallography and Photonics,” Russian Academy of Sciences, Moscow, 119333 Russia *e-mail: [email protected] **e-mail: [email protected] Received February 19, 2018; revised February 19, 2018; accepted April 19, 2018
Abstract—Nd1 – yEuyF3 – y crystals (y is the molar fraction of EuF2; 0 ≤ y ≤ 0.15) with a tysonite (LaF3) structure have been grown from melt by the Bridgman technique; their optical transmission spectra and dependences of the lattice parameters, density, refractive index, and fluorine-ion conductivity on the Eu2+ content have been investigated. Nd1 – yEuyF3 – y crystals are single-phase; they are crystallized into the trigonal system (sp. gr. P 3c1, Z = 6) at y ≤ 0.12, whereas at y = 0.15 one can observe stabilization of a high-temperature tysonite α-phase (sp. gr. P63/mmc, Z = 2). The crystals under study are transparent in the IR range up to 12 μm. The EuF2 doping into the NdF3 matrix leads to a monotonic decrease in the density and refractive index. The dependence of the conductivity on the composition σdc(y) has a nonmonotonic character. The Nd0.97Eu0.03F2.97 crystal has the maximum σdc value at 293 K (2.0 × 10–4 S/cm). Its charge-carrier concentration is nmob = 5.8 × 1020 cm–3 and the carrier mobility is μmob = 2.2 × 10–6 cm2/(V s) (at 293 K). The σdc value for Nd0.97Eu0.03F2.97 is smaller (by a factor of 2.5) than that for Ce0.97Sr0.03F2.97, which has the best electrolytic characteristics among R1 – yMyF3 – y solid electrolytes (M = Ca, Sr, or Ba). DOI: 10.1134/S1063774519020147
INTRODUCTION High-temperature chemistry of rare-earth element (REE) fluorides, which are prone to the formation of unusiual oxidation state R2+ (Sm2+, Eu2+, Yb2+), is not as well developed as chemistry of REE trifluorides RF3. The reason is that chemical RF2 for these REEs must be prepared under laboratory conditions because of the absence of commercial products. The degree of REE reduction is difficult to control; therefore, the question of crystal chemical composition should always be solved. Fluoride materials containing europium ions in the lowest oxidation state (2+) are interesting as fluorineconducting solid electrolytes (FSEs), the deviation from stoichiometry in which can be achieved by heterovalent isomorphic replacement of REE R3+ with Eu2+ [1–3]. This study is aimed at searching for and optimizing the composition of FSEs involving difluoride EuF2, without going into the details of its partial oxidation. At the low oxidation state of Eu2+, R1 – yEuyF3 – y
crystals can be considered as pseudobinary ones. A large fraction of europium oxidiz
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