Cation Vacancy Complexes Research Articles

Energy and Entropy Parameters for Vacancy Formation and Mobility in Ionic Crystals from Conductance Measurements

A new technique for the evaluation of defect parameters (enthalpy and entropy of creation of a pair of vacancies, enthalpy and entropy of migration of these vacancies, and the free energy of formation of divalent cation—cation vacancy complexes) has been developed. Unlike all previous approaches this method of analyzing conductance data does not make any approximations other than those inherent in the Teltow nearest-neighbor model. The application of this technique to new measurements of the specific conductance of single crystals of pure KCl and of KCl+SrCl2 is described.

Paramagnetic Resonance of Mn++ in NaN3, KN3, and RbN3

The paramagnetic resonance of Mn++ in single crystals of NaN3, KN3, and RbN3 is studied at 9.1 Gc/sec as a function of crystal orientation in the magnetic field. In KN3 and RbN3 containing Mn++ the crystalline electric field is the resultant of a large axial field in the [001] direction plus a small cubic field. The g values for Mn++ in KN3 are g∥ = 1.9961±0.0005, g⊥ = 1.9878±0.0050; and for Mn++ in RbN3 g∥ = 2.0005±0.0005, g⊥ = 1.9971±0.0050. The axial electric field parameter D is —534±3.0 G for KN3 and —278±3.0 G for RbN3 at 25°C. The cubic field parameter a0 is 10±0.5 G for KN3 and 8.7±0.5 G for RbN3. The Mn++ hyperfine coupling constants A and B are —89.7 and —91.1±0.5 G, respectively, in KN3. In RbN3, A and B are —88.0 and —88.9±0.5 G, respectively. The large magnitudes of D, A, and B allow the forbidden ΔM = ±1, Δm = ±1 transitions to be intense. Two inequivalent sites result from the displacement of the Mn++ from a cation site toward a bound nearest-neighbor cation vacancy. For an unheated crystal of NaN3, the main Mn++ resonance is a single broad line at g = 1.95±0.01. Heating the NaN3 crystal changes this broad line into multiple sets of 30-line spectra. At 25°C these sets of 30-line spectra decay slowly and the original broad line regrows. A similar effect previously found for Mn++ in NaCl has been attributed to mobility of Mn++-cation vacancy complexes. One type of Mn++ spectrum in NaN3 is due to the vacancy-associated complex and another type is due to the dissociated or excited complex. For both types of spectra g∥ = g⊥ = 2.001±0.002 and A≈B = 87±1.0 G. For the dissociated Mn++ complex state the spectrum has axial symmetry about the c axis and D = —240 G. For the vacancy-paired Mn++ spectrum an additional rhombic distortion occurs, and D = —265 and E = +57 G. The variation of linewidth with temperature is used to show that the KN3 spectra result from charge compensation by vacancy pairing. Additional effects produced by vacancy jumping are noted. Differences between the high-temperature properties of Mn++ in NaN3 and KN3 are related to cation size effects. A low-temperature line broadening of the Mn++ spectrum in KN3 and RbN3 is reported, and the similarity to the Mn++ resonances in solution-grown KCl and KBr is noted.