Carl W. F. T. Pistorius

Melting Curve of Ice VII to 200 kbar

The melting curve of ice VII has been determined to 200 kbar, where ice VII melts at 442°C. The curve can be given as P=−301+2.215T−5.231×10−3T2+4.367×10−6T3 where P is the freezing pressure in kilobars at T°K. The results agree within experimental error with the Simon equation in the form (P−21.54)/5.03=(T/354.8)5.135−1, but the power series fit is better. The main uncertainty in the results is the actual pressure values at the highest pressures.

Potential Field and Force Constants of Octahedral Molecules

The force constants of 10 molecules and 5 ions of the type XY6 belonging to the Oh symmetry are calculated from the observed frequencies by Wilsons F—G matrix method, using Raman and infrared data. Although only one force constant is neglected, imaginary results were found in the cases of MoF6 and WF6. Possible causes for this are discussed.

Phase Diagrams of H2O and D2O at High Pressures

The VIII/VII phase boundaries of solid H2O and D2O were studied by means of differential thermal analysis. The thermal hysteresis of the transition decreases with increasing pressure, but the presumed equilibrium temperatures are independent of pressure to 40 kbar. The transition is of the first order, and the average transition temperature is −3°C in both cases. The ice VI/VII transition pressure at 25°C is 21.39 ± 0.05 kbar, in good agreement with other recent studies, but considerably lower than Bridgmans value. The heavy ice VI/VII and VI/VIII transition lines are located ∼1.4 kbar below the ice VI/VII and VI/VIII transition lines. The heavy ice VI/VII transition at 25°C occurs at 19.90 ± 0.07 kbar. Bridgmans melting curves of ice VI and ice VII are shown to be correct. The melting curve of heavy ice VI is ∼2°C above that of ice VI, but the heavy‐ice VI/VII/liquid triple point is located at 78°C, 20.6 kbar as compared with 81.6°C, 21.97 kbar for H2O. The melting curve of heavy ice VII appears to hav...

MELTING AND POLYMORPHISM OF LIBH4 TO 45 KBAR

a new dense phase LiBHi III appears. The III/IV and IV/I transition lines rise with pressure to meet near 29.6 kbar. A further new phase LiBHi V appears at the V/III/I triple point at 37.3 kbar, 214.5°C. LiBHi V is slightly denser than LiBHi III at the III/V transition. The melting curve of contained LiBHi I extrapolates to ~315°C at atmospheric pressure, and is terminated at the V/I/liquid triple point at 29.3 kbar, 487.5 °C. It is suggested that LiBHi V is a disordered form of LiBHi III, and that LiBHi I is a disordered form of LiBHi II. None of the phase encountered can be expected to be closely related to the NaCl-lïke structure of NaBHi.

Polymorphic transitions of the alkali chlorides at high pressures to 200°C

Abstract The temperature dependence of the polymorphic transitions from the NaCl type to the CsCl type structures were determined to 200°C for NaCl, KCl and RbCl. The latent heats at 25°C are 660 ± 370 cal mole , −10 ± 8 cal mole and 130 ± 20 cal mole , respectively. LiCl has no transition up to 38 kilobars. The transition in KCl is very well suited for pressure calibration points in the region 50°–1040°C.

Polymorphic transitions of the alkali bromides and iodides at high pressures to 200°C

Abstract LiBr and LiI have no transitions to 40 kbar in the temperature range 20–150°C. The temperature dependence of the transitions from the NaCl to the CsCl type structure was determined to 200°C for KBr, RbBr, KI and RbI. Easily measurable slopes were found for the RbBr and RbI transition lines in spite of the fact that Bridgman reported these transitions to be independent of temperature. The transition pressures of KBr and KI are almost independent of temperature and are thus exceptionally well suited for calibration of high-pressure equipment at elevated temperatures. These transitions are sluggish near room temperature, but above 50°C the hysteresis drops sharply to about ±30 bar. This is among the lowest hystereses encountered. Extrapolation of our high temperature data to room temperature probably yields data of higher precision on KBr and KI than those submitted by Kennedy and LaMori. NaBr and NaI appear to have very sluggish transitions near 10–14 kbars and 200°C. The evidence is not conclusive, however.

Phase Diagrams of Sodium Tungstate and Sodium Molybdate to 45 kbar

The phase diagrams of Na2WO4 and Na2MoO4 have been determined to 40–45 kbar. Two new high‐pressure phases have been found for Na2WO4. The 440°C transition in Na2MoO4 consists of two transitions very close together. A further new high‐pressure phase has been found. Crystallographic data are given for Na2WO4 I and Na2MoO4 III, both of which are orthorhombic, Space Group Pnam. The phase diagrams of Na2WO4 and Na2MoO4 are strikingly similar, and there are reasons for believing that every known polymorph of Na2WO4 has an isostructural counterpart in the phase diagram of Na2MoO4.

Evaluation of Force Constants of Plane XY3 Molecules from Vibrational Data

All five force constants of the most general harmonic potential field are evaluated for 4 molecules and 4 ions of the plane XY3 type (symmetry group D3h) by means of Wilsons F—G matrix method. In the cases where no isotopic data are available, this is made possible by the introduction of an approximation from Heath and Linnetts OVFF treatment. Semiquantitative corrections are made for anharmonicity and for the product rule wherever possible. In the case of BF3, the results are compared with those of a previous worker.

Phase Diagrams to High Pressures of the Univalent Azides Belonging to the Space Group D4h18‐I4 / mcm

The high‐pressure phase diagrams of CsN3, TlN3, RbN3, and KN3 were studied by means of differential thermal analysis and volume displacement. These substances decompose explosively near ∼ 460°C even at high pressures. CsN3, TlN3, and RbN3 each have a new high‐pressure transformation with a volume change of ∼ 3.5% near 6 kbar at 25°C. The transition pressures increase with temperature. In the case of CsN3 this transition line meets the tetragonal/bcc transition line at a triple point at 173°C, 8 kbar. The resulting CsN3 III/bcc phase boundary was followed to 37 kbar, where it is terminated by explosive decomposition. The TlN3 tetragonal/bcc boundary was followed to 20 kbar, where it is also terminated. The melting curves of KN3 and TlN3 were followed only to ∼ 5 kbar, where they are terminated.

Unsymmetrical Friction and Pressure Calibration in Internally‐Heated Piston‐Cylinder Type High‐Pressure Devices

It is shown that the procedure of evaluating pressure losses in internally‐heated piston‐cylinder devices by assuming symmetrical friction is in error below ∼500°C. Previous results may be in error by as much as ∼5 kilobars at 40 kilobars and 25°C. A method of calibration is described which evaluates such unsymmetrical pressure losses.