Mojtaba Kahrizi

Thermal expansion and phase transitions in some tetra-alkylammonium metal chlorides☆

Abstract Using thermal expansion measurements on single crystals of compounds with general formula A2BX4 (A=tetra-alkylammonium group, B=metal ions, X=halogen), several structural phase transitions have been detected. The nature of these transitions and their dependence upon the metal ions is discussed.

Structural phase transitions in (C2H5)4N)2CdX4 and ((CH3)4N)2CdX4 compounds with X = Cl, Br

Abstract We report thermal expansion measurements on single crystals of ((CH 3 ) 4 N) 2 CdCl 4 , ((CH 3 ) 4 N) 2 CdBr 4 , ((C 2 H 5 ) 4 N) 2 CdCl 4 and ((C 2 H 5 ) 4 N) 2 CdBr 4 using capacitance dilatometry. Several phase transitions have been detected for each compound on warming and cooling the samples in the temperature range 77–300 K.

Observation of transitions to spin‐slip structures and splitting of the Néel temperature of holmium in magnetic fields

We present the results of magnetization measurements on single‐crystal holmium using a SQUID magnetometer in the temperature range from 4 to 140 K in magnetic fields up to 5.5 T. In low fields (0.01 T) the magnetization versus temperature data show a spiral to conical transition at Tc=16 K and the Neel temperature at 132 K. In addition, we observe new anomalies in the temperature dependence of the magnetization along the a, b, and c axes at 20, 24, 42, and 98 K. These new anomalies appear at the same temperatures as observed by Bates et al. [J. Phys. C 21, 4125 (1988); 21, 4113 (1988)] in ultrasonic velocity measurements on holmium. These anomalies could be accounted for within the frame work of the ‘‘spin‐slip’’ model of Gibbs and co‐workers. In the c axis magnetization we observe a splitting of the Neel temperature in magnetic fields greater than 0.5 T. The H‐T phase diagrams of the magnetic phases of holmium for fields in three directions (along the a, b, and c axes) are presented.

Phase transitions and thermal expansion in langbeinite type compounds

Abstract We report thermal expansion measurements using capacitance dilatometric methods on several polar and non-polar compounds of the langbeinite family, from liquid helium to room temperature. In K 2 Mn 2 (SO 4 ) 3 and Tl 2 Cd 2 (SO 4 ) 3 new thermal expansion anomalies indicative of phase transitions were detected. Similarly, two phase transitions were detected for the first time at 227.8 K and 330.8 K respectively in Tl 2 Mg 2 (SO 4 ) 3 . In (NH 4 ) 2 Mg 2 (SO 4 ) 3 two phase transitions were discovered at 220 K and 241 K with decreasing temperature, although with increasing temperature only one phase transition could be detected.

Investigation of the helimagnetic phases of holmium in a c -axis magnetic field

Neutron diffraction measurements have been made in the (h0l) plane of holmium in a c‐axis magnetic field of 3 T. Thermal expansivity measurements have also been made by capacitance dilatometry. The main focus of our investigation has been a region about 2 K wide near 96 K, where the helimagnetic structure locks into a commensurate state. We have also been able to observe the ‘‘2‐τ’’ satellites (at twice the fundamental magnetic ordering q vector) at temperatures as high as 3 K below the Neel transition. In the region between the lock‐in transition and the Neel point, the intensities of these satellites show interesting behavior with temperature and may be showing effects due to the anomalies which have been identified in our magnetization measurements.

THERMAL EXPANSION MEASUREMENTS ON SOME TETRA-ALKYLAMMONIUM METAL BROMIDES

Abstract We have measured the thermal expansion of single crystals of several tetra-alkylammonium metal bromides. The number and nature of the phase transitions found in these compounds is compared to the situation in the corresponding chlorides.

Dilatometric investigations on single crystals of the isomorphs — CaCd(OAc)4.6H2O and CaCu(OAc)4.6H2O☆

Abstract Capacitance dilatometry was used to investigate the thermal expansion of single crystals of CaCu(OAc) 4 .6H 2 O and CaCd(OAc) 4 .6H 2 O in the temperature range 77–295 K. The measurements were made along various crystal axes of the samples. No phase transition was detected in the case of CaCu(OAc) 4 .6H 2 O, but in the case of CaCd(OAc) 4 .6H 2 O a first order phase transition was observed at 141.7+/−0.1 K. From the relative thermal expansion of the various crystal axes of this compound, we conclude that it undergoes a transition from a tetragonal structure at room temperature, to one of at most monoclinic symmetry, as the temperature drops below 141.7K.

CRITICAL THERMAL-EXPANSION OF DYSPROSIUM

Abstract Two crystals of Dy have been examined using capacitance dilatometry, to clarify the nature of the Neel transition. The data suggest that the transition is very weakly first order. The critical exponent observed before the first-order transition intervenes is 0.1, differing in sign from theoretical predictions.

Exploration of the phase diagram of holmium in a magnetic field using neutron diffraction and thermal expansion measurements

Abstract We report the analyses of measurements of the intensities and positions of the satellites arising from the helimagnetic structure of holmium, and the associated anomalous thermal expansion, in the region within 40 K of the Neel temperature in an applied field of 3 T along the c -axis. While the decrease in the satellite intensity, which is observed while the wave-vector is locked into the commensurate value of 0.25, may be related to larger extinction effects in a phase in which the moment per cell is the same throughout the sample, the increases in intensity observed as the temperature passes into and leaves the region of commensurability, are much less easy to explain. The thermal expansion coefficient is found to mirror these effects, which is in accord with the suggestion of Plumer that the temperature dependence of the wave-vector is related to the strain dependence of the exchange interaction.

Spurious oscillatory effects in ultrasonic attenuation

Abstract The origin of frequently reported oscillatory effects in ultrasonic attenuation, in the vicinity of phase transitions, or any region of rapidly varying ultrasonic velocity or thermal expansion, is shown to arise from well known interference effects between an echo and a portion of the transducer which is ringing due to imperfect bonding.