Magnetic and crystallographic investigations of the inorganic spin-Peierls system α′-NaV2O5

Journal of Magnetism and Magnetic Materials 177-181 (1998) 743 745

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Journal of magnetism and magnetic materials

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Magnetic and crystallographic investigations of the inorganic spin-Peierls system at'-NaV205 M. Weiden a'*, R. Hauptmann a, C. GeibeP, M. K6ppen a, J. MiJller a, M. Lang a'b, F. Steglich a'b, N. Weiden c, M. Fischer d, P. Lemmens d, G. G f i n t h e r o d t d a Technische Physik, TH Darmstadt, Hochschulstr. 8, 64289 Darmstadt, Germany bMP1 chemische Physik fester Stoffe, Dresden, Germany CPhysikalische Chemie 111, Petersenstr. 20, 64287 Darmstadt, Germany d physikalisches Institut, R WTH Aachen, Templergraben 55, 52056 Aachen, Germany

Abstract

We present measurements on single- and polycrystalline samples of the spin-Peierls compound 0t'-NaV2Os. The transition temperature Tsp obtained in susceptibility measurements is strongly dependent on the Na-stoichiometry of the samples. The 0-values obtained in ESR measurements are close to O ~ 2. The crystallographic distortion due to the dimerisation at Tsp is seen in thermal expansion as well as in Raman light scattering measurements. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Peierls spin transition; Susceptibility - temperature dependent; Thermal expansion; Raman scattering

One of the unusual ground states encountered in quasi-one-dimensional spin systems is the spin-Peierls (SP) ground state, where a magneto-elastic coupling leads to a dimerisation of the spin chain and thus to the formation of a non-magnetic singlet below the transition temperature Tsp 1,1-3]. Up t o 1993, this transition has been observed in only a few organic compounds [4-6]. The discovery of an SP-state in the inorganic compound CuGeO3 [7] has renewed strong interest in this phenomenon: for the first time, detailed studies on single- and polycrystalline undoped and doped material were possible [8, 9]. Yet, despite intensive research for other compounds, CuGeO3 has remained the only known inorganic SP system. Within the end of 1996 and the beginning of 1997, Qt'-NaV205 was established as the second inorganic SP-system [10, 11]. This was proven by susceptibility measurements on a single crystal 1,11, 12] which showed the isotropic magnetic behaviour above and below the SP transition, by temperature-dependent X-ray measure-

*Corresponding author. Fax: +49 6151 164462; e-mail: [email protected].

ments which showed the crystallographic distortion due to the dimerisation [13], by inelastic neutron scattering which proved the singleWtriplet excitation 1-13], and by Raman measurements which showed the crystallographic distortions as well as several magnetic excitations below Tsp [11]. The preparation of a'-NaV205 is quite difficult due to the reactivity of the Na and the sensitivity to the oxygen content. In addition, the stoichiometry, in particular, the Na content, has great influence on the physical properties. In Fig. 1, we compare the susceptibility of several polycrystalline samples of ct'-NaxV205 with slightly different Na-content x. The stoichiometric sample (x = 1) has the highest transition temperature Tsp = 34 K. Already a very slight deficiency of Na reduces the SP transition temperature very drastically: with x = 0.98, Tsp is decreased to 30 K. Also, the drop of the susceptibility below Tsp is reduced, and an additional low-temperature Curie-tail due to unpaired V 4 + spins appears. At x = 0.96, only a very small anomaly around Tsp ~ 27 K is observed, higher Na-deficiency suppresses the SPtransition completely. The high-temperature part at T >~ 60 K of the susceptibility and consequently the exchange-coupling constant J remains uneffected within the range 0.95 ~< x ~< 1 (not shown). This drastic effect of

0304-8853/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PI1 S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 7 1 7-8

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M. Weiden et al. /Journal of Magnetism and Magnetic Materials 177-181 (1998) 743-745

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Na-deficiency on the SP-state is a consequence of the nonmagnetic V ~+ created inside the V~+-chains interrupting these chains. A similar effect is known from in-chain-doping in C u G e O 3 [9]. Besides the formation of the nonmagnetic singlet evidenced by the exponential decrease of the susceptibility, a crystallographic distortion is observed at the SP transition due to the dimerisation of the magnetic ions inside the chains. In Fig. 2, we show the first results of thermal-expansion measurements which were performed on a stoichiometric (x = 1) polycrystalline sample. In the volume-expansion coefficient fl versus T, a sharp transition at Tsp is observed. In addition, a change of slope at T = 31 K is observed, resembling the results obtained on CuGeO3, where a similar change of slope below Tsp occurs [,14]. ESR measurements were performed with a W-band spectrometer at r o o m temperature on a stoichiometric (x = 1) single crystal. The g-values obtained from a field sweep at constant frequency (v = 94.4 GHz) are g, = 1.978 and gc = 1.95 with the magnetic field along the aand c-axis, respectively. This very small anisotropy proves a ' - N a V : O 5 to be nearly an ideal isotropic Heisenberg system above Tse. Raman measurements offer a sensitive tool to investigate the crystallographic distortion below Tsp. Below Tsp, several folded phonons due to the doubling of the unit cell as well as several magnetic excitations are observed [-10] in measurements on a stoichiometric (x = 1) single crystal. In Fig. 3a, we show the temperature dependence of an SP-induced mode at 65 c m - ~ obtained in (aa)-polarisation: starting at Tsp, a strong increase in intensity towards lower temperature is observed. The origin of this excitation is probably magnon-scattering from the nonmagnetic singlet ground state to the excited triplet. This interpretation is based on its energy which is

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T (K) Fig. 1. Susceptibility of polycrystalline 0t'-NaxV205 as a function of temperature. Already a small Na-deficiency reduces the SP-transition temperature by several K. Also, the drop of the susceptibility is much smaller. With x = 0.95, the SP-state is completely suppressed (not shown).

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T(K) Fig. 2. Volume-expansion coefficient ofa stoichiometric (x = 1) polycrystalline sample. A sharp anomaly at Tsp = 34 K and an additional change of slope at T = 31 K are observed. Inset: relative change of the volume.

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Fig. 3. Raman measurements on a single crystal (x = 1) in (aa)(upper part) and (bb)- polarisation (lower part) as a function of temperature. The modes at 65 and 130 cm -1 are assigned to magnetic Raman scattering. The intensity of the folded phonon at 670 cm- 1 increases below Tsp. very close to the singlet-triplet excitation observed in inelastic neutron scattering at 9 meV - 72 c m - x Also, the intensities of two other modes appearing below Tsp at 105 and 130 c m - 1 are strongly temperature dependent. D u e to their respective line shape and energy, the m o d e at 105 c m - 1 is assigned to an induced phonon, while the m o d e at 130 c m - ~ is assigned to two-magnon scattering. Both magnetic contributions at 65 and 130 c m - 1 are also observed in (bb)-polarisation parallel to the V 4 +-chains and in crossed polarisation. This points to a less pronounced one-dimensional character of the spin system. In Fig. 3b, the development of a folded phonon at 670 c m - 1 in (bb)-polarisation can be observed. To summarise, we discussed the dependence of the magnetic properties on the N a stoichiometry in the

M. Weiden et al. / Journal of Magnetism and Magnetic Materials 177-181 (1998) 743 745

inorganic spin-Peierls c o m p o u n d 0t'-NaV2Os. We also discussed the crystallographic distortion at the SPtransition which causes a sharp anomaly in thermalexpansion coefficient. In addition, the development of gap-induced and lattice-dimerisation-induced R a m a n scattering was presented as a function of temperature. This work was supported by SFB 252, SFB 341 and B M B F FKZ.13N6586.

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