ELSEVIER Journal of Alloys and Compounds 219 (1995) 69-72

Journal o| AUD~

AND C£~)L~IDS

Charge density waves in some Nb and Ta chalcogenides

A. Prodan a, Vo Marinkovi6 a,b, F.W. Boswell c, J.C. Bennett d, M. Rem~kar a a j. Stefan Institute, University of Ljubljana, Ljubljana, Slovenia

b Department of Metallurgy, University of Ljubljana, Ljubljana, Slovenia c Guelph-Waterloo Program for Graduate Work in Physics, Waterloo Campus, University of Waterloo, Waterloo, Ont., Canada

d Department of Physics, University of Alberta, Edmonton, Alta., Canada

Abstract

A brief description of the competing mechanisms important for the stability of composition- and temperature-dependent modulation phases of Nbl_xTaxTe4 is given, with some emphasis on the microstructural changes at the two high temperature phase transitions in TaTe4. The charge density wave sliding in NbSe3 is explained on the basis of a structural model which takes into account the easy switching between the two modulation wavevectors along the same trigonal prismatic columns. The dependence of the interpolytypic transitions and various charge density wave transition temperatures on the amount of intercalated Ag in 1T-TaS2 is discussed.

Keywords: Chalcogenides; Charge density waves; Niobium; Tantalum

1. Introduction

There are two conditions for the appearance of a structural distortion driven by a charge density wave (CDW) [1]. The first is connected with the shape of the Fermi surface, where a "nesting" instability can take _place for distortions with a modulation wavevector ~=2kF, while the second condition requires strong electron-phonon interactions such that the electron density modulation is followed by a structural distortion of the positive ion lattice.

In the present work our studies carried out with three representative systems are briefly described. The conditions for the appearance of various CDW phases in Nbl _xTaxTe4 and some microstructural features con- nected with the two high temperature phase transitions in TaTe4 are given first. Then the non-linear transport properties and CDW sliding in N b S e 3 a r e discussed in view of a possible structural instability. Finally, silver intercalation into the van der Waals gaps of the two- dimensional TaS2 and its influence on the formation of various structural polytypes and CDW phases is described.

2. The Nbl_xTaxTe4 system

This system is of interest for several reasons. The average structure is relatively simple, there is a complete

range of solubility between N b T e 4 and TaTe4, the modulation superstructures are stable up to temper- atures where the crystals disintegrate and there is a series of composition- and temperature-dependent phase transitions.

After the average structures had been determined [2,3] and it had been shown that the room temperature (RT) superstructures were incommensurate (IC) in NbTe4 and commensurate (C) in TaTe4 [4], a number of papers were published on electron microscopy and diffraction, X-ray structural analysis, various crystal- lographic approaches and theoretical work based on the Landau theory of phase transitions. This entire work is summarized in a collection of review articles where all relevant original references can also be found [5]. Thus, instead of going into particular aspects of the problem, a few general remarks only will be given here.

The various IC and C phases observed in the Nbl_xTaxTe4 system are a result of two competing mechanisms. With all modulation columns equivalent and with all intra- and intercolumn Te-Te distances kept constant, energy reduction is achieved through the formation of transition metal atom triplets along the chains and through a phase shift as large as possible between the CDWs of adjacent columns. These two requirements cannot be achieved simultaneously and the prevailing mechanism will change with composition

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70 A. Prodan et al. / Journal o f Alloys and Compounds 219 (1995) 69-72

and temperature. Since the RT modulation in NbTe4 is IC, triplets only cannot form, but the neighbouring CDW columns are set out of phase. In contrast, in RT TaTe4 the C modulation forms Ta triplets shifted in phase'by 2~/3 with respect to the nearest neighbours, which enlarges the modulation unit cell base.

Two important features should be mentioned in this connection. First, the transitional phases between the RT IC and the low temperature (LT) lock-in phase of NbTe4 are characterized by small additional per- turbations to the RT breathing mode, which are equiv- alent to some elements of the RT TaTe4 superstructure. Second, the modulation wave vector 3 is changed step- wise with composition in Nbl_xTaxTe4 and also varies when Ti, Zr or V is substituted for Nb or Ta.

We have been particularly interested lately in the microstructural features of both C-to-C and C-to-IC high temperature (HT) phase transitions in TaTe4. It was shown that on heating the crystal through the transition from RT C to HT C, the straight type I antiphase boundaries (APBs) associated with a dis- placement vector /~=½(110) gradually disappear on account of an increase in width of the remaining type II ones associated with a displacement vector /~2=~(302). The transition into the HT C phase is initiated at these APBs and spreads through the entire crystal. This coincides with a reduction of the RT C superstructure unit cell. Cycling through this transition reveals a pronounced memory effect on the APBs if the specimen is not heated too far over the transition temperature and as long as the cycling is slow. On further heating, the remaining APBs visible in the satellite dark field images gradually evolve into multiple lines. Their density continues to increase with tem- perature, until near 550 K a regular array of fringes about 200/~ apart is observed. This process takes place through nucleation of the sixfold "discommensuration dislocations", similar to what Mahy et al. [6] found in NbTe4. These defects are mobile, so that finally all domains contain a dense array of fringes, which coincides with the HT C satellites of TaTe4 turning IC (although only about 0.6%). On cooling, this situation persists to near 450 K, where a direct transformation into the RT superstructure takes place. During this transition C strips nucleate and expand, eventually covering the entire crystal with the exception of the remaining type II APBs. These different mechanisms on heating and cooling account for the large thermal hysteresis.

3. Sliding charge density waves in NbSe3

NbSe3 is one of the few CDW-driven modulated structures where non-linear transport properties have been observed and thoroughly studied (see Refs. [7,8] and references therein). Its RT monoclinic average

structure is formed of three slightly different pairs of Nb-centred Se trigonal prisms. The formation of trigonal prismatic chains along/~ and the cross-over linking of eight-coordinated Nb atoms to the neighbouring Se columns along ~ result in a two-dimensional character of the compound [9]. According to Refs. [10-13], two IC CDWs appear on cooling from RT: at T1 = 144 K the first is formed along type III ("yellow") columns with ql--(0.0, 0.243, 0.0) and at T2=59 K an additional one appears along type I ("orange") columns with 32 = (0.5, 0.263, 0.5). These distortions were shown to be localized at the two types of columns, where they slide under the application of an electric field gradient (0.1 V cm -~ or more) [14]. In contrast, the type II ("red") columns with almost equilateral bases remain practically undistorted.

Although the modulated structures were studied re- cently by synchrotron radiation [15] and in spite of numerous electrical, optical and magnetic measurements [16,17] as well as nuclear magnetic resonance [18-20] and scanning tunnelling microscopy studies [21-24], the problem of CDW sliding is not completely understood. Pinning at defects and impurities certainly strongly influences the threshold voltage and the actual sliding [17], but there must be further reasons which make the CDWs unstable in some cases only. Another open question is what causes the enlargement of the 32 modulation unit cell basis as compared with that of the average structure while the one of 3~ modulation remains unchanged.

The basic hypothesis of our suggested model is that on cooling, the modulation indeed takes place along the type III columns first and at still lower temperatures also along the type I ones. However, regardless of the column type, once the modulation is initiated, it takes on a random distribution of both 31 and q2 modes which interchange easily. The disorder allows two en- ergetically equivalent out-of-phase ~)2 settings (in ad- dition to a single 31 setting) along the type III columns and vice versa along the type I columns. If this disorder takes place on a much smaller scale as compared with the coherence region, the contribution to the reciprocal space of 32 modulation along type III columns and of 31 modulation along type I columns is practically lost. The model can be formally tested within a long- period superstructure approximation with an (a + ~) x 58/~ x (e - ~) unit cell, with the y components of the two modulation modes taken a s 31 = 14/58 and q2 = 1 5 / 5 8 , with the disorder introduced by overlapping various modulation modes along the same columns and with the modulation amplitudes taken from the recent (3+2)-dimensional synchrotron radiation refinement [15].

The phase relationships between CDWs along various types of columns cannot be unambiguously determined. The origin of this problem lies in a selected transfor-

A. Prodan et al. / Journal of Alloys and Compounds 219 (1995) 69-72 71

mation of parts of the real space into certain parts of the reciprocal space. CDWs along type III columns contribute practically only to the intensities of those satellites which lie on lines connecting the main re- flections, while CDWs along type I columns cause only those satellites which lie on lines exactly halfway between the first. This means that practically the diffraction pattern is independent of the phase relationship between CDWs along type III and type I columns. In addition, while all adjacent ql columns can be set out of phase, the t~2 CDW arrangement between different columns must include smaller phase shifts also, which will result in the enlarged unit cell. If the modulation along all columns is considered as a superposition of both ql and q2 modes, with all inter- and intracolumn pair phase shifts set to be ~r/2 with respect to the large 58b periodicity, different phase shifts and thus different modulation unit cell bases for the two modes result automatically. This is a consequence of the ~1 and ~2 modes being represented by an even and an odd function respectively.

The suggested model certainly requires a detailed structural analysis to be confirmed or rejected. However, it explains CDW sliding on the basis of a structural instability, which has only a minor impact on the diffraction pattern and is not in contradiction with other measured physical properties.

The time-dependent strings and moir6-1ike fringes ("twinkling"), which Fung and Steeds [25] observed during transmission electron microscopy below both transition temperatures seem to be in accord with our model. They already suggested a wavevector variation to be a possible origin of the peculiar observations in spite of observing on the whole the ~1 satellites only between the two transition temperatures. The apparent contradiction between their observation and expecta- tions can be understood if the contribution of one or possibly more components is indeed averaged out be- tween domains which are small compared with the diffraction coherence regions.

The model also explains the controversial scanning tunnelling microscopy (STM) image of N b S e 3 at 4.2 K [21-24]. We believe the figure shows a certain domain with both strongly modulated lines of atoms belonging to the type I and type III columns, in spite of being modulated with the same q2 periodicity and thus out of phase as compared with the adjacent pairs of lines. This is also in agreement with the calculation of Ren and Whangbo [26], who showed that the modulation intensities along different columns in the STM images of N b S e 3 should be in the order/type In > / type 1 > / type lI.

4. Charge density wave phases in AgxTaS2

TaS2 crystals consist of S-Ta-S sandwiches with covalent bonding among atoms inside sandwiches and

with weak van der Waals bonding between them. The coordination of Ta atoms is either octahedral (AbC) or trigonal prismatic (AbA) and the sandwiches can be stacked in pure octahedral (1T), pure trigonal pris- matic (2H) or mixed (4Hb, 6R) coordination polytypes [271.

The 1T-TaS2 polytype exhibits a wide range of CDW phases [28]. These include the LT C phase 1T3 (below 180 K), the HT IC phase 1T1 (above 350 K) and the intermediate nearly C phases 1T (between 220 and 350 K) and 1T2 (between 280 and 350 K). Heating and cooling cycles are different in so far as the 1T phase does not appear during cooling. The mixed stacking in 6R- and 4Hb-TaS2 (BcB CaB CaC AbC AbA BcA and AcB AcA BcA BcB respectively) lowers all transition temperatures, so that the transition from nearly C to HT IC takes place at about 315 K [27].

We evaporated silver 0.5-5 nm thick on to freshly cleaved (00.1) surfaces of 1T-TaS2 to study its influence on the formation of various CDW phases. Regardless of the substrate temperature (between 300 and 570 K) and the amount of deposited silver, the latter almost entirely intercalated into the van der Waals gaps of the substrate during the deposition itself. A charge transfer between the intercalated Ag and the partly occupied d : energy bands of TaS2 takes place and the remaining Ag ions order into either a 2a or a ~/3a superstructure depending on the local Ag concentration. This is in accord with observations in electrochemically intercalated 2H-TaS2 [29].

The intercalated Ag influences the stability of various CDW phases in two ways, both lowering the transition temperature.

First, a partial overlap of the Ta ~2 orbitals from adjacent sandwiches bridged by Ag ions causes elon- gation of the Ta-centred octahedra, which eventually flop over entire sandwiches into the trigonal prismatic coordination. Since the local concentration of Ag is variable, 6R and 4Hb polytypes result, with the latter preferred at lower Ag concentrations. The correlation between adjacent sandwiches is partly lost [27] and satellites forbidden in 1T-TaS2 appear. Simultaneously, the CDW modulation amplitudes and thus the cor- rugations observed in STM images are reduced as compared with pure 1T-TaS2.

Second, intercalated Ag reduces various CDW tran- sition temperatures by filling up the d : energy band with donor electrons. Electron transfer into the d2 band reduces the necessity of CDW formation and thus splitting of the energy bands [30]. As a result, the HT IC phase was observed at RT. For even larger Ag contents a complete disappearance of CDW satellites took place as a consequence of the CDW onset tem- perature being pushed below RT.

72 A. Prodan et al. / Journal of Alloys and Compounds 219 (1995) 69-72

Acknowledgements

Valuable discussions with Dr. A. Budkowski from the Department of Physics at the Jagellonian University in Krakow, Poland as well as the assistance of Mrs. Z. ~kraba in technical work and crystal growth are gratefully acknowledged.

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