Ambient X-ray Diffraction Research Articles

Clamping effect on temperature-induced valence transition in epitaxial EuPd2Si2 thin films grown on MgO(001)

Bulk ${\mathrm{EuPd}}_{2}{\mathrm{Si}}_{2}$ show a temperature-driven valence transition of europium from $\ensuremath{\sim}+2$ above 200 K to $\ensuremath{\sim}+3$ below 100 K, which is correlated with a shrinking by approximately 2% of the tetragonal crystal lattice along the two $a$-axes. Due to this interconnection between lattice and electronic degrees of freedom the influence of strain in epitaxial thin films is particularly interesting. Ambient x-ray diffraction (XRD) confirms an epitaxial relationship of tetragonal ${\mathrm{EuPd}}_{2}{\mathrm{Si}}_{2}$ on MgO(001) with an out-of plane $c$-axis orientation for the thin film, whereby the $a$-axes of both lattices align. XRD at low temperatures reveals a strong coupling of the thin film lattice to the substrate, showing no abrupt compression over the temperature range from 300 to 10 K. Hard x-ray photoelectron spectroscopy at 300 and 20 K reveals a temperature-independent valence of $+2.0$ for Eu. The evolving biaxial tensile strain upon cooling is suggested to suppress the valence transition. Instead temperature-dependent transport measurements of the resistivity and the Hall effect in a magnetic field up to 5 T point to a film thickness independent phase transition at 16 to 20 K, indicating magnetic ordering.

Phase development in ZnO varistors

Based on a typical ZnO varistor composition (97·0 mol.-% ZnO, 1·0 mol.-% Bi2O3, 1·0 mol.-% Sb2O3, 0·5 mol.-% MnO and 0·5 mol.-% Co3O4), phase development of the ZnO varistor during sintering has been investigated using in situ high temperature X-ray diffraction up to 900°C, and conventional ambient X-ray diffraction for samples sintered at 900°C to 1250°C. The results indicate that α-Bi2O3 can be detected until 700°C; the pyrochlore phase can be detected in the samples heat treated at 700°C and up to 1250°C; the spinel phase is present at and >900°C. However, the main phases in the varistor are established by 950°C. By this temperature, the essential microstructure features are formed, and the varistors exhibit non-linear electrical properties, with a non-linear coefficient α of 35 and breakdown field of 8000 V cm−1. With increasing sintering temperature, both the α value and breakdown field decrease.

Relation between martensitic transformation temperature range and lattice distortion ratio of NiMnGaCoCu Heusler alloys

In order to study the relation between martensitic transformation temperature range ΔT (where ΔT is the difference between martensitic transformation start and finish temperature) and lattice distortion ratio (c/a) of martensitic transformation, a series of Ni46Mn28−xGa22Co4Cux (x = 2–5) Heusler alloys is prepared by arc melting method. The vibration sample magnetometer (VSM) experiment results show that ΔT increases when x > 4 and decreases when x < 4 with x increasing, and the minimal ΔT (about 1 K) is found at x = 4. Ambient X-ray diffraction (XRD) results show that ΔT is proportional to c/a for non-modulated Ni46Mn28−xGa22Co4Cux (x = 2–5) martensites. The relation between ΔT and c/a is in agreement with the analysis result obtained from crystal lattice mismatch model. About 1000-ppm strain is found for the sample at x = 4 when heating temperature increases from 323 K to 324 K. These properties, which allow a modulation of ΔT and temperature-induced strain during martensitic transformation, suggest Ni46Mn24Ga22Co4Cu4 can be a promising actuator and sensor.

Characterization and phase transitions of (Bi,Pb) 2Sr 2Ca 2Cu 3O x–Ag composite powder obtained by spray pyrolysis

(Bi,Pb) 2Sr 2Ca 2Cu 3O x –Ag (BPSCCO/Ag) composite precursor powder was prepared by spray pyrolysis. The as-prepared and thermally treated (750 °C (1 h) and 780 °C (3 h)) powders were characterized by transmission electron microscopy, ambient X-ray diffraction, In situ X-ray diffraction, magnetic susceptibility measurement and differential thermal analysis (DTA). DTA curve exhibited two strong endothermic peaks above 790 °C, one very wide with the onset at 790 °C and the second at 840 °C. In situ X-ray diffraction measurements strongly supported the view that the first endothermic peak was due to Ca 2PbO 4 decomposition and a transient liquid formation through PbO–CuO–Ag eutectic. The second endothermic peak can be explained by onset of Bi-2212 peritectic melting that began at 840 °C with local formation of Bi-2201 intergrowths. The melting was complete at 875 °C.