The conductivity of polycrystalline ammonium chloride (NH₄Cl), fluoride (NH₄F), and bromide (NH₄Br) under high uniaxial pressures from 15 to 50 GPa in the temperature range 77 to 400 K was studied.
For all three materials a sharp decrease in resistivity by orders of magnitude from the value more than 10⁸ Ohm to kOhms was observed at some threshold pressure P_c being about 42, 27 and 15 GPa for NH₄F, NH₄Cl and NH₄Br, respectively. Similar sharp transition was observed near P_c in the temperature dependence of resistivity. P_c values correspond to a stationary state, which was reached after sufficiently long exposure of samples under stress. The time of the pressure treatment necessary to stabilize this jump in resistance is quite different for these three materials. Initial loading of the samples by the pressure of 50 GPa causes the change in the resistance only after the exposure under stress during a month for NH₄F, about 10 days for NH₄Cl and seconds for NH₄Br. There exist the correlation between the times of such a treatment, as well as values of P_c, and the density of the materials (the atomic weight of halogens F, Cl and Br). Relaxation times of conductivity are different at increasing and decreasing pressure. The relaxation time is essentially larger near P_c.
At the pressures and temperatures above this transition points, a hysteresis in the resistance of all three materials in dependence on temperature was observed, indicating the existence of intermediate states. The magnitude of the hysteresis decreases with the number of cycles of applying and removing pressure. All ammonium halides show metal-like behaviour under high pressures similar to that of alkali halides.
This work was supported by CRDF grant REC-005 for Ural Center of Research and Education "Advanced materials" and RFBR grant 01-03-96494.

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