Low temperature X-ray investigation of NH₄Br

Abstract

156SHORT COMMUNICATIONS present measurements. The ratio ux/ue however, was found to rise from 0.033 to 0.048 with the increased accelerating potential and so the beam homogeneity appears to be substantially independent of tube voltage.In Table 1 the observed flux changes at 30 kV are compared with those predicted on the basis of (1) for 30 kV and 12 kV for a beam of 1 mm diameter that subtends 16 minutes of arc (0.26 ° ) at the tube focus. The results show that for intensity measurements of 1% accuracy the take-off angle should never be less than about 5 ° for Cu Kct radiation.References ALEXANDER, L. E. • SMITH, G. S. (1962 The lattice parameters of NH4Br have been measured as a function of temperature from 5 to 150°K, a range which includes the region of the transition between cubic and tetragonal ordered phases. The lowtemperature CsCl-type phase exhibits normal thermal expansion up to ~ 108°K on warming, while the tetragonal diffraction pattern persists down to ~80°K on cooling. The present data are combined with previous measurements between 150 and 300°K to yield an overall picture of the variations associated with changes in ordering.Four different structural modifications of NH4Br are known to exist at low pressures, and a fifth modification of unknown structure has recently been observed above 2000 atm (Garland & Young, 1968). At one atmosphere and high temperatures NHaBr has a disordered NaCl-type structure, but it undergoes a first-order transition at 410°K to a disordered CsCl-type structure. [P6yh6nen, Mansikka & Heiskanen (1964) have measured the lattice parameters between 297 and 467°K.] This modification undergoes a cooperative 2 transition at 234.5 °K to an ordered tetragonal structure which is closely related to the CsC1 structure (Levy & Peterson, 1953). At lower temperatures there is a sluggish first-order transition to an ordered CsCl-type structure. Associated with this latter phase change is a marked hysteresis: the transition occurs at ~ 78 °K on cooling and at ~ 108 °K on warming (Stephenson & Adams, 1952;Sorai, Suga & Seki, 1965). The order-disorder referred to above involves the relative orientations of adjacent NH4 + ions. Below 234.5°K there is a long-range order in these orientations although this ordering will not be the same in the tetragonal and CsCl-type structure.Hovi, Heiskanen & Varteva (1965) have carried out a detailed X-ray determination of the lattice parameters of NHaBr as a function of temperature between 150 and 300 °K. The object of the present investigation was to confirm the cubic structure of the low-temperature phase, to determine the lattice parameter as a function of temperature, and to investigate by means of X-ray diffraction the region of the order-order transition at ~ 100 °K. To achieve these goals, we have obtained data between 5 and 150°K.

Magnetic Ordering of Ammonium Cations in NH4I, NH4Br, and NH4Cl

“…The ammonium halides are known for their ability to be supercooled and superheated by large temperature ranges. ,,,,− This explains the large hysteretic regions observed in the warming and cooling χ­( T ) curves. From these curves, the increase at T β‑δ_Cl = 135 K in χ­( T ) (Figure ) was due to the β phase of NH 4 Cl remaining supercooled all the way down to 135 K before transitioning into the δ phase.…”

“…The A and B configurations form columns along the c axis. For NH 4 Br and NH 4 Cl, a δ phase (IV) exists at the lowest temperatures at ambient pressure (space group P :3 m ), which also has a CsCl-like structure, but the orientations of all of the NH 4 + align parallel to each other, viz., either all A or all B. The NH 4 + tetrahedra undergo C 3 and C 2 rotations, which are essentially simultaneous “jumps” of three and four protons by 120 and 180°, respectively, to each other’s sites (gray arrows in Figure a).…”

The thermodynamics of mixed crystals of ammonium chloride and ammonium bromide. II. An analysis of the heat capacity of ammonium chloride, ammonium bromide, and an approximately equimolar solid solution of these salts

“…Nissila & Poyhonen (1973) measured V dilatometrically on either side of the y-P transition, their results for the P-phase being reasonably consistent with the earlier findings of Jaakkola et al (1968) and Poyhonen et al (1964). Bonilla et al (1970) determined V by X -ray diffraction experiments for temperatures from 150 to 5 K, thus covering phases 8 and y. We have combined the values of a derived from their work and those obtained from the Finnish studies to give the smoothed figures recorded in table 1.…”

Calculation of the free energies for the solid phases of ammonium halides

“…By means of definitions of f, equation (7), and k, equation (12), 2 , equation (15), and 2 , equation (16), can be expressed, respectively, as…”

“…Studies on the -phase transitions of NH 4 Cl [1][2][3][4][5][6][7][8][9][10][11] and NH 4 Br [5,[12][13][14][15][16][17][18][19][20][21][22] have been reported previously, and have been reviewed in our earlier studies [11,22]. For NH 4 I, the -phase transition has also been investigated using various techniques [23][24][25][26][27][28][29][30][31][32], as reviewed in our more recent study [32].…”