Investigation of the Rotary Lattice Mode in R2PtCl6 Compounds. II. From a Study of the 35Cl Nuclear Quadrupolar Spin–Lattice Relaxation

1971 ◽  
Vol 49 (19) ◽  
pp. 2389-2395 ◽  
Author(s):  
Robin L. Armstrong ◽  
Douglas F. Cooke
Keyword(s):  
Relaxation Time ◽  
Pressure Dependence ◽  
Lattice Relaxation ◽  
Lattice Relaxation Time ◽  
Spin Lattice Relaxation ◽  
Published Data ◽  
Spin Lattice Relaxation Time ◽  
Spin Lattice ◽  
Lattice Mode ◽  
Temperature And Pressure

Measurements of the temperature and pressure variation of the 35Cl nuclear spin–lattice relaxation time in Rb2PtCl6 and Cs2PtCl6 are reported. The spin–lattice relaxation time is measured at atmospheric pressure for temperatures from 60 to 500 K and at four temperatures between 290 and 380 K for pressures to 5000 kg cm−2. Previously published data for K2PtCl6 are also included in the discussion. The Van Kranendonk theory of nuclear quadrupolar relaxation forms the basis of the analysis. The rotary lattice mode frequencies are deduced; they are of approximately the same magnitude and increase in the same sequence as the frequencies deduced from nuclear quadrupole resonance frequency measurements and from infrared and Raman data. An analysis of the pressure dependence of the spin–lattice relaxation time data yields order of magnitude pressure coefficients for the rotary mode frequencies. Finally, a thermodynamic analysis, which takes specific volume effects into account by incorporating both the temperature and pressure dependence of the data, is presented.

The Observation of a Displacive Phase Transition in (NH4)2PtBr6

1973 ◽  
Vol 51 (8) ◽  
pp. 781-786 ◽  
Author(s):  
Maria Wiszniewska ◽  
Robin L. Armstrong
Keyword(s):  
Phase Transition ◽  
Relaxation Time ◽  
Structural Phase ◽  
Lattice Relaxation ◽  
Lattice Relaxation Time ◽  
Spin Lattice Relaxation ◽  
Tetragonal Symmetry ◽  
Spin Lattice Relaxation Time ◽  
Spin Lattice ◽  
Lattice Mode

Measurements of the 79Br nuclear quadrupole resonance frequency and spin–lattice relaxation time in a polycrystalline sample of (NH4)2PtBr6 from 4 to 450 K are reported. The frequency data indicate the occurrence of a structural phase transition at 58 K. The spin–lattice relaxation time data exhibit a distinct minimum, also at 58 K. The data are discussed in terms of a model previously introduced to account for phase transitions from cubic to tetragonal symmetry in R2MX6 compounds. A point charge calculation provides an explanation of the observed frequency splitting in terms of the alteration in the neighboring ion contribution to the electric field gradient at the 79Br nuclear sites. This alteration results from a small distortion of the cages defined by the NH4 ions and an accompanying rotation of the PtBr6 octahedra situated within these cages. The temperature dependences of two different average rotary lattice mode frequencies are deduced; in each case the observed behavior supports the hypothesis that it is the rotary lattice mode which softens thereby bringing about the change in structure.

NUCLEAR SPIN-LATTICE RELAXATION TIME IN TIN

1978 ◽  
Vol 39 (C6) ◽  
pp. C6-1215-C6-1216
Author(s):  
H. Ahola ◽  
G.J. Ehnholm ◽  
S.T. Islander ◽  
B. Rantala
Keyword(s):  
Relaxation Time ◽  
Nuclear Spin ◽  
Lattice Relaxation ◽  
Lattice Relaxation Time ◽  
Spin Lattice Relaxation ◽  
Spin Lattice Relaxation Time ◽  
Spin Lattice ◽  
Nuclear Spin Lattice Relaxation

Pressure dependence of iodine nuclear relaxation in rubidium iodide

1978 ◽  
Vol 56 (10) ◽  
pp. 1386-1389
Author(s):  
Marie D'Iorio ◽  
Robin L. Armstrong
Keyword(s):  
Relaxation Time ◽  
Lattice Relaxation ◽  
Low Pressure ◽  
Lattice Relaxation Time ◽  
Lattice Defects ◽  
Spin Lattice Relaxation ◽  
Polymorphic Phase Transition ◽  
Spin Lattice Relaxation Time ◽  
Spin Lattice ◽  
Pressure Phase

The pressure-induced polymorphic phase transition at about 4 k bar in rubidium iodide was studied using nuclear magnetic resonance. The signature of the structural transition is a loss of echo intensity which presumably is due to an increase in the number of lattice defects as a result of the transition. The ratio of the spin–spin relaxation times of the iodine nuclei in the two phases is in agreement with the ratio predicted by a second moment calculation. The actual experimental values, however, are considerably smaller than the theoretical predictions signifying the migration of lattice defects. Estimates of the iodine spin–lattice relaxation time at atmospheric pressure indicate the necessity to include both an anharmonic Raman contribution and a covalency factor. The change in spin–lattice relaxation time with pressure as measured in the low pressure phase is dominated by the change in the lattice parameter. At the critical pressure the spin–lattice relaxation time decreases by a fractional amount which is approximately equal to the fractional volume change characterizing the transition. The pressure derivative of the spin–lattice relaxation time in the high pressure phase is nearly equal to that in the low pressure phase.

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