Paramagnetic NH3Cl and NH3Br Color Centers in Irradiated Ammoni

Abstract

Radiation-induced NH 3 C1 and NH 3 Br defect centers have been studied in ammonium halide single crystals by EPR and optical spectroscopy. The defects are most likely formed by removal of H° from an ammonium ion with subsequent bonding of NH 3 + to X~ along a (111) direction. In the NH3CI center, NH3 4 " is a nearly planar group loosely bonded to a Cl~~; in the NH 3 Br center there is more tendency toward sp z hybridization of the N orbitals, and stronger covalency within the nitrogen-halogen bond. The molecular orbital description of these centers shows a strong similarity to that of mixed VK color centers in the alkali halides. Optical absorptions due to the NH3CI and NH 3 Br centers were observed at 3.31 and 3.14 eV, respectively. The NH 3 C1 defects were observed to undergo thermally induced reorientations which are strongly influenced by the ordering of the surrounding ammonium ions at low temperature. INTRODUCTIONT HE ammonium halides are a suitable system for the extension of studies of radiation-induced color centers which have in the past been studied primarily in the alkali halides. 1 Recent electron-paramagnetic-resonance and optical-absorption experiments in the ammonium halides have led to the identification of two color centers which are also found in the alkali halides: (1) the F center, which is an electron trapped at a negative ion vacancy 2 ; (2) the VK center, a hole shared by two nearest-neighbor halide ions. 3 -4 A third center has been identified as NH 3 X, where X represents a halogen atom. 5 ' 6 In this paper we give a fuller description of the structure of this center in ammonium chloride, deuterated ammonium chloride, and ammonium bromide single crystals. The kinetics of the reorientation behavior, dealt with briefly here, will be described more completely in a subsequent publication. 7 CRYSTAL STRUCTUREBoth NH4CI and NH 4 Br have the CsCl structure at room temperature, 8 as shown in Fig. 1. Below 242.8°K in the chloride, 234.5°K in the bromide, both materials undergo a X transition of the order-disorder type involving the relative orientations of adjacent ammonium ions within the lattice. 9 The tetrahedral NH 4 + ions can be oriented in two equivalent positions of minimum electrostatic potential within the cube of nearestneighbor halide ions. In the completely ordered state 1 of NH 4 C1, all NH 4 + ions have the same relative orientation, for a given domain, with respect to the crystallographic axes; the space-group symmetry is T d l {PiZm). ND4CI has similar structural changes. The NH 4 Br crystal structure becomes tetragonal below the X point, though this is the result of a slight change of 0.4% of the lattice parameter along one cube direction. Additionally, the Br~ ions are displaced about 3.0% from planes perpendicular to the distortion axis, lying alternately in positive and negative directions along the tetragonal axis. The space-group symmetry is Dih 7 (P/nmm). In the ordered NH 4 Br crystal, adjacent NH 4 + ions in a plane perpendicular to the distortion axis have opposite orie...

X-ray stability and degradation mechanism of lead halide perovskites and lead halides

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Radiation Damage Studies of Co2+ and Ni2+ Doped NH4Cl Crystals

“…NH,C1 is another centre [5] produced in NH,C1 crystals on X-irradiation at 77 K. It has an optical absorption band a t 375 nm which lies in the same region as the UV band of the V,-centre. Its ESR peaks also overlap with those of the V,-centre.…”

“…Its ESR peaks also overlap with those of the V,-centre. Hence NH,Cl was studied [5] by warming the crystal, X-irradiated at 77 K, little above 155 K and then cooling it back to 77 K. The ESR study of NH,CI has indicated an axially symmetric hyperfine pattern with the symmetry axis along a (111)-direction. This is stable up to 220 K only.…”

Electrical conductivity in NH4Cl and ND4Cl single crystals

“…This effect may be seen directly in the electron paramagnetic resonance spectra of ND 3 Cl color centers [2] in deuterated single crystals. If these crystals are exposed to the atmosphere for short periods of time, proton hyperfi ne interactions from hydrogen which has exchanged with deuterium atoms in the ammonium group become pronounced in the spectra.…”

“…It was noted, during studies on radiation-induced point defects in ammonium halide single crystals [1,2], that hydrogen migrates fairly rapidly through these crystals at room temperature. This effect may be seen directly in the electron paramagnetic resonance spectra of ND 3 Cl color centers [2] in deuterated single crystals.…”