Physics of the Solid State, Vol. 42, No. 12, 2000, pp. 2177–2183. Translated from Fizika Tverdogo Tela, Vol. 42, No. 12, 2000, pp. 2113–2119. Original Russian Text Copyright © 2000 by Mikheeva, Panova, Teplov, Khlopkin, Chernoplekov, Shikov. METALS AND SUPERCONDUCTORS Thermodynamic and Kinetic Properties of an Icosahedral Quasicrystalline Phase in the Al–Pd–Tc System M. N. Mikheeva, G. Kh. Panova, A. A. Teplov, M. N. Khlopkin, N. A. Chernoplekov, and A. A. Shikov Russian Research Center Kurchatov Institute, pl. Kurchatova 1, Moscow, 123182 Russia e-mail: [email protected] Received April 26, 2000 Abstract—The properties of a quasicrystalline phase in the Al–Pd–Tc system are studied for the first time. X-ray investigations demonstrate that the quasicrystalline phase in the Al70Pd21Tc9 alloy has a face-centered icosahedral quasi-lattice with parameter a = 6.514 Å. Annealing experiments have revealed that this icosahedral phase is thermodynamically stable. The heat capacity of an Al70Pd21Tc9 sample is measured in the temperature range 3–30 K. The electrical resistivity and magnetic susceptibility are determined in the temperature range 2– 300 K. The electrical resistivity is found to be high (600 µΩ cm at room temperature), which is typical of qua- sicrystals. The temperature coefficient of electrical resistivity is small and positive at temperatures above 50 K and negative at temperatures below 50 K. The magnetic susceptibility has a weakly paramagnetic character. The coefficient of linear contribution to heat capacity (γ = 0.24 mJ/(g-atom K2)) and the Debye characteristic tem- perature (Θ = 410 K) are determined. The origin of the specific features in the vibrational spectrum of the qua- sicrystals is discussed. © 2000 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION of radiation safety rules. Attempts to produce the Al Pd Tc quasicrystalline alloy have failed [8]. It is well known that quasicrystals belong to a spe- 70 20 10 cial class of solids lacking the periodic order inherent The quasicrystalline icosahedral phase in the Al– in usual crystal systems. However, unlike amorphous Pd–Tc system was first prepared in our previous work materials, quasicrystals are not disordered systems. [9] by the conventional solidification technique. Quasicrystals are characterized by a specific long- The aim of the present work was to investigate range order—the so-called “quasi-periodic” order. The experimentally the thermodynamic and transport prop- long-range noncrystalline orientational order (for erties of this phase. example, the long-range order that corresponds to a fivefold rotational symmetry, as is the case in icosahe- dral quasicrystals) was revealed by electron diffraction 2. SAMPLE PREPARATION AND STRUCTURAL and x-ray diffraction analyses [1]. INVESTIGATIONS The first quasicrystalline materials (aluminum- An ingot of the Al70Pd21Tc9 alloy was produced by enriched transition metal alloys with an icosahedral melting the initial high-purity metals: Al (99.999%), Pd structure) were thermodynamically unstable. Subse- (99.99%), and Tc (99.98%). The melting was carried quently, stable icosahedral quasicrystals were obtained out in an electric arc furnace with a water-cooled cop- in the Al–Cu–Li, Al–Cu–Fe–TM (TM = Fe, Ru, and per bottom. Permanent tungsten electrodes were used. Os), Mg–Zn–Ga, Mg–Zn–R (R = Y, Dy, Tb, Ho, and In order to ensure alloy homogeneity, a workpiece was Er), and Al–Pd–TM (TM = Mn and Re) systems [2–7]. turned over then remelted; this process was repeated four times. The workpiece thus prepared was annealed Investigation into the ternary Al–Pd–TM systems ° (where TM = Mn or Re) showed that the icosahedral at a temperature of 940 C for 3 days under vacuum and then was quenched in water. The sample was light gray phase is observed for compositions in which the ratio × × between components is close to 7 : 2 : 1. In Group VII in color with a metallic luster. A small bar 1.65 1.7 of the periodic table, technetium is placed between 5.35 mm in size was cut from the workpiece; moreover, manganese and rhenium. This gives grounds to assume a powder suitable for the structural investigation was that the quasicrystalline icosahedral phase also exists in also prepared from the same workpiece. The procedure the Al–Pd–Tc system. However, until recently, infor- of sample preparation was described in detail in [9]. mation on the Al–Pd–Tc system was unavailable. This The structure of the alloy prepared was investigated was primarily due to the fact that technetium has no sta- by x-ray powder diffraction on a Philips ADP-10 x-ray ble isotopes and is of limited occurrence. Moreover, diffractometer (CuKα radiation, graphite monochroma- experimenting on this element requires the observance tor) intended for measurements of radioactive samples. 1063-7834/00/4212-2177 $20.00 © 2000 MAIK “Nauka/Interperiodica” 2178 MIKHEEVA et al. 1.0 18.29 20.32 7.11 6.9 8.12 52.84 0.5 38.61 40.64 70.113 Intensity, arb. units 0 10 20 30 40 50 60 70 80 90 2θ, deg Fig. 1. X-ray diffraction pattern of the Al–Pd–Tc powder. Dark arrows show the peaks attributed to the face-centered icosahedral structure. Light arrows indicate the peaks assigned to the hexagonal compound Al3Pd2. Indices of the icosahedral phase are dis- cussed in the text. Figure 1 displays the x-ray diffraction pattern of an indices, N and M, according to the Cahn scheme [10]. annealed sample of the Al70Pd21Tc9 alloy. This pattern It should be noted that the presence of the (7, 11) peak is identical to the x-ray diffraction patterns of the Al– confirms the fact that our quasicrystalline phase is the Pd–Mn and Al–Pd–Re quasicrystalline face-centered face-centered icosahedral phase. This structure can be icosahedral phases. As can be seen, the diffraction pat- considered a six-dimensional superstructure in a prim- tern exhibits additional peaks, which are not attributed itive hypercubic lattice in real space with the six- to the quasicrystalline face-centered icosahedral phase dimensional lattice parameter aP, which is half as much and, most likely, correspond to the Al3Pd2 compound as the parameter aF of the face-centered lattice [2]. In involved in the form of an impurity phase. Furthermore, the diffraction patterns experimentally obtained for the a halo assigned to the varnish coating of the sample is face-centered icosahedral phases, the reflections asso- observed in the diffraction pattern in the range 2θ = ciated with the superstructural ordering (N = 4n + 3, 18°–30°. where n is the integer) are often very weak compared The peaks shown in Fig. 1 that correspond to the to the reflections of the primitive lattice. Correspond- quasicrystalline phase are marked by arrows with two ingly, these reflections are usually indexed using the indices of the primitive lattice with the lattice parame- ter aP = aF/2. Q, Å–1 For the icosahedral peaks, the wavevector magni- 6 tude is given by the relationship Al–Pd–Tc 2π N + Mτ Qpar = ------ -------------τ-----, (1) aP 2 + 4 τ where aP is the quasi-lattice parameter and = ( 5 – 1)/2 is the golden section. Figure 2 shows the wavevector Q, which was deter- mined for each x-ray diffraction peak indexed in the 2 icosahedral phase, as a function of the product QparaP calculated by formula (1). The dependence Q(QparaP) for all the indexed icosahedral reflections is depicted in Fig. 2 together with the straight line describing this dependence. The mean absolute deviation of peak posi- 0 10 20 30 40 tions (3 × 10–3 Å) is considerably less than the full mean QparaP width at half-maximum (2 × 10–2 Å), which is a weighty argument in support of the applicability of our Fig. 2. Wavevector Q for each x-ray diffraction peak attrib- indexing procedure described above. uted to the face-centered icosahedral phase as a function of the product QparaP calculated for the icosahedral structure. The quasi-lattice parameter aP can be determined, Wavevector Q for each peak was determined using the with a high accuracy, from analysis of the x-ray powder weighted mean wavelength of the Kα doublet. diffraction patterns at large angles with respect to the PHYSICS OF THE SOLID STATE Vol. 42 No. 12 2000 THERMODYNAMIC AND KINETIC PROPERTIES 2179 incident x-ray beam. This is explained by the fact that ρ(T)/ρ(300 K) the large Bragg angles are very sensitive to small vari- 1.0 ations in the quasi-lattice parameter, as is evident from the expression for the derivative of the quasi-lattice θ: parameter aP with respect to the Bragg angle θ θ daP/d = –aP cot . 0.9 Averaging of the data on QparaP/Q over the diffraction angle range 63° < 2θ < 90°, which involves three reflec- ± tions, gives the quasi-lattice parameter aP = 6.514 0.004 Å. This value is very close to the quasi-lattice parameter a = 6.451 Å for the Al–Pd–Mn icosahedral P 0.8 quasicrystal [11]. 0 100 200 300 It should be noted that certain x-ray powder diffrac- T, K tion peaks, which cannot be indexed in the icosahedral Fig. 3. Temperature dependence of the electrical resistivity structure, are most probably attributed to the Al3Pd2 of the Al–Pd–Tc quasicrystalline sample in the temperature compound. This compound crystallizes in a hexagonal range 2–300 K. structure of the Al3Ni2 type with the unit cell parame- ters a = 4.217 Å and c = 5.166 Å (Fig. 1). The heat capacity of the sample was measured by A cast sample of Al Pd Tc contained a mixture of 70 21 9 the adiabatic technique with pulsed heat input [13].