Abstract
Pure HfW2O8- and Ln3+-containing solid solutions, Hf1−xLnxW2O8−x/2 (Ln = Eu, Tm, Lu), were synthesized using the hydrothermal method. The lanthanide ions were selected based on the differences between their ionic radii. A content of the Ln3+ ions in the range of 0.01–0.15 mol with a step of 0.02 was used for Hf1−xLnxW2O8−x/2 preparation, although the main research was performed on x = 0.01 and 0.05 samples because of an inhomogeneity detected by powder X-ray diffraction (XRD) when the content of Ln3+ was above 0.07–0.09 mol. X-ray diffraction measurements were supported by Raman and infrared spectroscopy. A new band in the Raman spectra of the samples with 0.05 mol Ln3+, as well as a red shift of the most intensive band (assigned to valence stretching of W-O-W bonds) as a result of the Ln3+ presence, was detected. The Scanning Electron Microscopy and Transmission Electron Microscopy micrographs revealed well-crystalized microcrystals with lengths in the range of 2–5 μm, with larger interplanar distances, measured in the solid solutions of the same crystal plain. The alpha-HfW2O8 → beta-HfW2O8 order-to-disorder phase transition was followed by high temperature XRD, and its reversibility was evident. The influence of the Ln3+ both on the unit cell parameters of the solid solutions and on the temperature of phase transition and on the coefficient of thermal expansion, CTE, was observed. A band gap energy in the range of 2.8–3.1 eV for pure HfW2O8 and for the solid solutions Hf1−xLnxW2O8−x/2 (x = 0.01 and 0.05) was determined.
Highlights
Materials which contract upon heating, i.e., with negative thermal expansion (NTE), can play the role of thermal-expansion compensators, so they are considered important in the development of composites with adjustable thermal expansion [1–3]
ZrW2O8 goes through an order–disorder change at about 440 K (167 ◦C) from a metastable, low-temperature cubic phase with a coefficient of NTE of −9 × 10−6 K−1 to a metastable, high temperature cubic phase with a coefficient of NTE of −5 × 10−6 K−1 [8,10]
Calorimetric studies show that the phase transition temperature is 24 K higher for HfW2O8 than for ZrW2O8, which is probably due to the stronger chemical bond of Hf-O than Zr-O [12]
Summary
Materials which contract upon heating, i.e., with negative thermal expansion (NTE), can play the role of thermal-expansion compensators, so they are considered important in the development of composites with adjustable thermal expansion [1–3]. Examples of these materials include various silicates [4], cyanide-bridged compounds [5], nickel-based perovskite oxide [6], and many others, reviewed in [7]. The introduction of ions with different radii and charges can lead to a disorder in the crystalline structure of the tungstates and, to changes in their properties as detected by us for limited concentration ranges of Eu(III) on NTE and the phase transformation temperatures of ZrW2O8 [14]. In an attempt to expand the research, in this work, the influence of lanthanide ions Eu3+, Tm3+, and Lu3+ on the properties of HfW2O8 was followed by synthesis and characterization of solid solutions of the type Hf1−xLnxW2O8−x/2
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