ISSN 1062�8738, Bulletin of the Russian Academy of Sciences. Physics, 2012, Vol. 76, No. 3, pp. 342–344. © Allerton Press, Inc., 2012.Original Russian Text © G.V. Tikhomirova, A.V. Teben’kov, Ya.Yu. Volkova, A. N. Babushkin, 2012, published in Izvestiy Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2012,Vol. 76, No. 3, pp. 391–393.

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INTRODUCTION

Ammonium halides are analogs of alkali metalhalides, in which structural transitions under highpressure have been studied in detail, along with the rel�evant variations of optical and electrical properties,including the occurrence of metal�like states [1, 2].

In ammonium halides, the role of alkali metal isplayed by the (NH4)

+ ion. Various orientations of thision in the lattice lead to orientational transitions that donot occur in alkali metal halides. The variety of phasetransformations and complicated intramoleculardynamics of ammonium halides have attracted greatattention from reserarchers for many years [3–7].Structural studies of these materials have been per�formed mainly at pressures below 10 GPa [6, 7]. Theconduction of ammonium halides at high pressures wasstudied in [8, 9].

The aim of this work is to investigate the influenceof high pressures of up to 50 GPa on the electric con�ductivity of ammonium halides, and to determine theconditions and characteristic times of the generationof various high�pressure phases as a function of pres�sure treatment duration and the various sequences ofits application. The conductivity and magnetoresis�tance of NH4I are studied at pressures above 10 GPafor the first time.

EXPERIMENTAL

Our measurements were performed in a high�pres�sure cell (HPC) with diamond anvils made of carbon�ado synthesized polycrystalline diamond [10]. Theseanvils are good conductors and can be used as electriccontacts on a specimen. The resistance of short�cir�cuited anvils is several Ω and varies weakly with tem�perature. The method allowed us to study one and thesame specimen upon consecutive pressure increasesand decreases, and to hold it under a load over a longperiod of time. The investigated specimens obtained

by compression in the HPC, were about 0.2 mm indiameter and from 10 to 30 μm in thickness. The mea�surements were performed in the linear part of cur�rent�voltage characteristic.

RESULTS AND DISCUSSION

The phase transitions in ammonium halidesinduced by high pressure were found, which were evi�dent as sharp changes in conductivity. The character�istics of these transitions (the jumps in resistance, thegrowth of fluctuations near the transition pressures,and baric hysteresis) are typical for first�kind phasetransitions.

At low pressures, the resistance of the materialsstudied was higher than 100 MΩ. Upon a pressureincrease, the resistances of all the investigated ammo�nium halides dropped sharply after reaching a certaincritical value Pc2. Upon a subsequent pressuredecrease, we observed a reverse transition to a statewith high resistance at critical pressure Pc1 < Pc2. As anexample, Fig. 1 depicts the baric dependences of resis�tance of NH4Cl for various cycles of pressureincrease/decrease.

Similar sharp (stepwise) transitions and hysteresiswere observed in the temperature dependences ofresistance R(T) at pressures close to the critical values.The values of the loop of baric and temperature hyster�eses of resistance fell with an increase in number ofcycles of pressure loading and unloading and/or theperiod of holding under pressure. It points that suffi�ciently long high�pressure treatment is required inorder to achieve a steady state. In certain cases, staticholding at a fixed pressure is not sufficient, and severalsubsequent cycles of pressure increase/decrease arerequired.

Earlier, we found that in NH4X (X = F, Cl, Br)ammonium halides, the critical pressures of transitionPc1 from the low� to the high�resistance state do not

High Pressure Phase Transformations in Ammonium Halides Emerged in Conductivity

G. V. Tikhomirova, A. V. Teben’kov, Ya. Yu. Volkova, and A. N. BabushkinUral Federal University named after the first President of Russia B.N.Yeltsin, Yekaterinburg, 620000 Russia

e�mail: Galina.tikhomirova@usu.ru

Abstract—Comparative studies of high pressure effect on the resistance of ammonium halides NH4X (X = F,Cl, Br, I) at 77–400 K are presented. Conditions and characteristic times of various high�pressure phases for�mation are determined as a function of the time of pressure treatment duration and temperature. Conductiv�ity and magnetoresistance of NH4I at pressures above 10 GPa are studied for the first time.

DOI: 10.3103/S106287381203032X

BULLETIN OF THE RUSSIAN ACADEMY OF SCIENCES. PHYSICS Vol. 76 No. 3 2012

HIGH PRESSURE PHASE TRANSFORMATIONS 343

depend on a specimen’s prehistory, correlate with theanion�to�cation distance, and are 40, 25–27, and15 GPa, respectively, for NH4F, NH4Cl, and NH4Br[8]. During our studies of the conduction of ammo�nium iodide NH4I, we discovered a similar transitionin the pressure range of 8–10 GPa.

The critical pressures of transition in ammoniumhalides were analyzed using equations of state bySchlosser, Ferrante, and Vinet [5]; and by Birch [6, 7].It was established that the value of critical pressure Pc1is inversely proportional to the anion�to�cation dis�tance (Fig. 2).

Figure 2 illustrates the critical pressure as a func�tion of distance rka. A similar dependence on rka with�out pressure is given for comparison.

It turns out that at critical pressures, the distancebetween halogen and an ammonium ion becomescomparable to halogen’s ionic radius. We may thusassume that the transition to the conductive state isrelated to the sharing of hydrogen bonds of ammo�nium. The linear dependence of critical pressure onthe anion�to�cation distance in the ammoniumhalides studied shows that the structural transforma�tions in these materials are probably of the same type.

It was found that the properties of ammoniumhalides depend strongly on the duration of pressuretreatment and the baric prehistory of specimen. Thetime of initial processing by pressure required for thestabilization of low�resistance state of NH4X wasdetermined, and was different for different ammo�nium halides. The time of such holding for ammo�nium fluoride was about a month at 50 GPa; forammonium chloride, it was about a week at 50 GPa.Upon holding under lower pressures (e.g., 44 GPa) thetime required for a specimen to transition to the con�ducting state rose to 17–20 days. For ammonium bro�mide, holding under high pressure was not required, asthis material transitioned to the conducting state upon

the initial application of pressure at around 22 GPa.Ammonium iodide transitioned to the conductingstate at pressures below 8 GPa. A correlation wasdetermined between processing time as well as criticalpressure and the ionic radiuses of halogens F, Cl, Br,and I.

CONCLUSIONS

Studies of magnetoresistance provide additionalconfirmation of the existence of phase transforma�tions at high pressures. The electric resistances ofspecimens were measured as a function of the mag�netic field (up to 1 T). For specimens of ammoniumchloride near the pressure of transition from the low�to the high�resistance state upon an increase in themagnetic field, the resistance rises at first. The depen�dence corresponds approximately to the classic(square�law) type. The resistance then starts to fall.Negative magnetoresistance was found. The depen�dence for a pressure of 50 GPa is typical for when thereis a gapless state [11]. The value of relative magnetore�sistance of NH4Br in magnetic fields of up to 0.4 T canbe as high as 50%. At pressures above 40 GPa, the neg�ative magnetoresistance up to 50% is also observed.

During our studies of conductivity and magnetore�sistance of NH4I, in addition to the aforementionedtransition at 8–10 GPa, peculiar features weredetected in the baric dependence of resistance in therange of 20–35 GPa. Application of transversal mag�netic field (up to 1 T) smoothes these peculiaritiesslightly and varies the type of baric hysteresis. Negativemagnetic resistance is observed. Such negative mag�netic resistance in a randomly heterogeneous mediumcould indicate weak localization near the critical pointof transition [12].

103

102

45353025

104

105

40P, GPa

50

106

107

R, ΩPc1 Pc2

Fig. 1. Baric dependences of resistance of NH4Cl at roomtemperature after two weeks of processing at high pressure(triangles) and upon a pressure decrease after subsequentprocessing under pressure for three months (circles).

45

25

3.63.23.02.6

35

15

2.8 3.4

P, GPa

rka, А

rka Pc( ) V/V0( )Pc

1 3⁄rka 0( )=

rka(0)

Fig. 2. Critical pressure as a function of the cation�to�anion distance in a deformed specimen (squares) and theliterature data under normal pressure.

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TIKHOMIROVA et al.

ACKNOWLEDGMENTS

The work was partially supported by the RussianFoundation for Basic Research, grant nos. 09�02�01316 and 10�02�96036�r_ural; and by the federal tar�geted program Scientists and Science Teachers of anInnovative Russia, 2009–2013.

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