Thermal conductivity of solid oxygen

Abstract

This paper reports the results of the first measurements of the thermal conductivity of the a, )3, and y phases of solid oxygen over the temperature range of 1-52 K. A simple qualitative analysis was performed to explain the observed anomalies in thermal conductivity, which manifested themselves by a jump at the a-fi transition, the anomalously weak temperature dependence in the p phase, and an increase of the conductivity with temperature in the y phase.PACS numbers: 66.70.+f, 31.70.Ks, 75.20.Ck, Solid oxygen, a unique crystal combining properties of a molecular crystal and a magnet, has been a subject of intensive experimental and theoretical studies during the last two decades (see, for example [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], and references therein). The following picture has emerged as a result of those studies. Oxygen exists under the equilibrium vapor pressure in three crystalline modifications. The orientationally ordered monoclinic low temperature a phase (C2/m) is a collinear two-sublattice quasi-2D antiferromagnet. The phase transition (7^ = 23.9 K) into the intermediate rhombohedral /3 phase (R3m) is associated with the transformation of the magnetic structure. The nature of this transformation is still a subject for discussion [13].Though none of the measured thermodynamic characteristics as well as the molar volume [5] show any jump at the a-fi transition point and, moreover, the heat of transition has not been detected in precise calorimetric study [3], the a-p transformation is beyond all doubts a first order phase transition. This conclusion follows, for instance, from the observation of hysteresis of the magnetization curves at the transition [11].The long-range magnetic order in /3-oxygen is most probably absent; however the short-range order with the correlation length of 5 A in three-sublattice or incommensurate helicoidal structures persists over the whole range of the existence of the phase [ 11,14,15].Both the low temperature phases have the same orientational structure in which the molecular axes are collinear and perpendicular to the close packed layers. The spectrum of elementary excitations in the a phase consists of (besides acoustic modes) two libron modes at 43 and 78 cm -1 [2] and two magnon ones at 6.4 and 27 cm -1 [1,4]. No magnon excitations were found in the p phase and there is one twofold degenerated libron mode at 50 cm" 1 [2].The p-y transition (7^=43.8 K) is accompanied by a radical rearrangement of the lattice, a considerable jump (5.4%) of volume [5], and high value of the latent heat of the transition [3]. The y phase has an eight-molecule cubic cell with an orientationally disordered structure with Pm3n symmetry. It exhibits paramagnetic properties with a quasi-ID magnetic short-range order [9,10].In this paper we report results of the first measurements of the thermal conductivity of the a, /?, and y phases of solid oxygen.The measurements of the thermal conductivity were carried out by the stationary heat flux method over a temperature ran...

Heat Transfer in γ-Phase of Oxygen

“…The dashed line shows the thermal conductivity measured under saturated vapor pressure (see [3]). It is seen that our results for nearly free sample 4 agree in the absolute value with the data of [3] but they exhibit a different temperature dependence. Sample 4 was grown under the pressure ~5…”

“…The thermal conductivity has a maximum in the α-phase at Т≈6 K, drops sharply on a change to the β -phase, where it is practically constant, jumps again at the β→γ transition and increases in γ-O 2 [3][4]. The experimental results are interpreted as follows.…”

“…This behavior is not quite clear in view of the fact that the molecular rotation in γ-O 2 is far from being free. Finally, above 35 K the phonon mean free path estimated from a simple gas-kinetic formula is close (≈ 10 -7 cm [3]) to the lattice parameter, which makes the applicability of the phonon concept questionable. The above features have attracted our interest and stimulated this investigation of the isochoric thermal conductivity of γ-O 2 .…”

“…True, under saturated vapor pressure such growth was observed, apart from oxygen, only in methane [8], where the molecular rotation becomes nearly free at the pre-melting temperatures. According to the calculation based on [3], γ-O 2 belongs to the molecular crystals which have the highest temperature coefficients of the growth of thermal conductivity f = (∂ ln Λ/ ∂ ln T) V (cf. f ≈3.5 for γ-O 2 , ≈0.35 for CH 4 (I) and ≈0.5 for CCl 4 (I) [7]).…”

“…The thermal conductivity of solid O 2 was investigated under saturated vapor pressure in the α-, β-and γ-phases in a temperature interval of 1-52 K [3][4]. The lowtemperature α-O 2 phase is ordered orientationally and magnetically.…”