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Diffusionless phase transformations of Ru2Si3, Ru2Ge3 and Ru2Sn3 II: electrical and magnetic properties
Journal of the Less-Common Metals, 71 (1980) Pl - P8 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
Pl
DIFFUSIONLESS PHASE TRANSFORMATIONS OF RuaSis, RuzGes AND RuaSns II: ELECTRICAL AND MAGNETIC PROPERTIES C. P. SUSZ* and J. MULLER Dkpartement de Physique de la Mat&e CH-1211 Geneva 11 (Switzerland)
Condense’e,
Universite’ de Gene’ve, 32 Bd. d’Yvoy,
K. YVON and E. PARTHi Laboratoire de Cristallographie aux RayonsX, CH-121 I Geneva 4 (Switzerland)
Universite
de Gene’ve, 24 Quai E. Ansermet,
(Received May 25,1979)
Summary The series of RusGes,Sn, and RusGes,Si, compounds (0 < 3c,y d 3) were investigated using ele@rical resistivity and magnetic susceptibility measurements. Except for the tin-rich compounds (x > 2) which are metallic, they are semiconducting of type n and are diamagnetic. The forbidden energy gaps for RusSis and RusGes, as measured in the high temperature modifications, are 0.44 and 0.34 eV respectively. The gap’increases discontinuously for all compounds while they undergo a first order phase transformation into the low temperature modification.
1. Introduction In two previous studies [ 1, 21 the crystal structures of RusSis, RusGes and RusSns have been examined by X-ray diffraction methods at various temperatures. The compounds, which belong to the class of so-called defect TiSis [ 31 or Nowotny chimney-ladder compounds [ 41, have been shown to undergo a diffusionless phase transformation from an orthorhombic centrosymmetric low temperature (LT) structure to a tetragonal noncentrosymmetric high temperature (HT) structure. According to X-ray measurements the temperatures at which the transformations occur decrease from about 1000 “c for RusSis to about 500 “Cfor RusGes and to below 0 C for RusSna. They are reversible and result from small displacements of the silicon, germanium and tin atoms with respect to the quasi-rigid array of ruthenium atoms forming a p-&r type of structure. Surprisingly the atomic displace*Present address: Usine genevoise de d6grossissage d’or, 4 Place des Volontaires, CH-1211 Geneva 11, Switzerland.
P2
ments are found to occur gradualIy over a wide temperature range, thus suggesting that the phase transformation is of second order. Because of the limited resolution of the X-ray powder data, however, no definite conclusion as to the absence of a small discontinuous change in the crystal structure during the transformation could be made. The purpose of the present study was to investigate the nature of this phase transition in more detail and to determine the electronic and magnetic properties of the binary compounds and their solid solutions. Of particular interest was the question of whether or not some of them show a semiconducting behaviour. In fact such a behaviour can be anticipated from a rigid band model which has been recently proposed by Jeitschko [ 5 J . This model, which is equivalent to the concept of a “partial valence electron concentration” as formulated by ParthQ [ 61, suggests that the electronic band struttures of all defect TiSis-related compounds are similar with respect to the transition metal (T) states. In particular, because of the existence of a forbidden energy gap, no more than.14 spin states per T atom site can be filled by a charge transfer from the silicon atom sites to the T atom sites. On the experimental side only a few compounds having the “magic” number of 14 valence electrons per T atom site have been studied, among which RuA12 (see ref. 5) and. Mn2,Si4, [ 71 are indeed semiconductors. However, it has been pointed out [ 83 that no compounds belonging to this st;ructural class have been found for which the total number VEC, of valence electrons per T atom site exceeds 14 by more than a few per cent (for Rhz7Ge2a VECT is 14.2). 2, Experimental The RusSia and RuaGes sainples were prepared by a two-step procedure. SmaIl pieces of ruthenium metal (99.9% purity) and germanium (99.988%) or silicon (99.99%) were melted together by inductive heating under vacuum in a water-cooled silver crucible. The samples were then remelted several times in an argon-filled arc furnace and annealed for 10 h at 1300 “c in a high vacuum furnace. Me~o~aphi~ ~sp~tion and X-ray powder diffraction analysis showed that the samples contained only a single phase. The tincontaining samples, on the contrary, had to be prepared in a different manner because Ru,Sna was found to form by a peritectoidic reaction [ 91. Well-mixed samples of ruthenium sponge and tin powder were heated in evacuated and sealed quartz tubes for as long as one month at temperatures around 1150 “C. After several cycles of grinding and annealing the resulting powder was compressed and melted in an argon-filled arc furnace. Despite several attempts to obtain a single phase the samples always contained some amount of RusSn7. The electrical resistivity measurements were performed on small bars of length 10 - 40 mm and cross section 1 mm X 1 mm. They were cut from the bulk samples by electro-erosion and were polished on all faces. For the low
P3
temperature measurements (1.2 K < T < 400 K) the electrical contacts were obtained by fixing four copper wires on these faces using a colloidal silver solution. For the high temperature measurements the contacts were made by spot welding four molybdenum wires in the case of the RupSna sample and by a simple clamping device for all other samples [ 91. This device consisted essentially of four molybdenum knife blades having a weight of 10 g each which ensured a constant pressure on the electrical contacts at all temperatures. The resistivity measurements were carried out in an induction furnace under high vacuum. In order to obtain an even temperature distribution the samples were surrounded by two concentric cylinders of tantalum foil which were closed at both ends.
3. Results and discussion 3.1. RuzSi, and RuPGea_,Si, (y = 1,2) The temperature dependence of the electrical resistivity is illustrated in Fig. 1. It can be seen that all the compounds definitely show a semiconducting behaviour and that there are some marked anomalies in the resistivity which occur in a temperature range in which structural phase transformations have previously been observed by X-ray diffraction [ 21. For instance in RuaSis the activation energy decreases very rapidly between 1100 and 1200 K, having a value of about 0.7 eV in the LT state and of about 0.4 eV in the HT state. Moreover the resistivity values obtained clearly depend on the thermal history of the sample. In fact for a given temperature the resistivity is always higher if the sample is heated up than if it is cooled down. This indicates the presence of a strong hysteresis effect which usually occurs during a first order phase transformation. When silicon is substituted by germanium the forbidden energy gap E, of the HT modification decreases. At the same time the transformation temperature to the LT modification also decreases. Interestingly enough E, is always significantly higher in the LT state than in the HT state. 3.2. RueGe, and RuzGes,Sn, (x = 0.25) It can be seen from the temperature dependence of the resistivity represented in Fig. 2 that RuzGes is semiconducting in both the LT and the HT modifications. However, the activation energy is smaller than that of the siliconcontaining compounds, being 0.34 eV in the HT state and 0.52 eV in the LT state. There is again a strong hysteresis effect between 700 and 1000 K, in accordance with the observation of a structural phase transformation in this temperature range. The semiconducting nature of RuaGe, was confirmed by a low temperature specific heat measurement. Between 1.5 and 5 K the specific heat varied as -yT + aT3, the measured Debye temperature being 435 f 10 K. The very small electronic term yielded -y = (29 f 3) X 10m6J K-’ (g atom))’ and
P4
pbO-hC?ld~
T( K)
\
‘ogP (arb.unit
5’ /
0.6
a7
0.6
103/T( K)
Cl9
10
0
2QO
400
600
6aJ
Ixx)
T( K)
Fig. 1. The logarithm of the electrical resistivity p as a function of the inverse temperature l/T for the semiconducting compounds RuzGea y Si, (y = 1, 2, 3). The hysteresis effect for RuzSia should be noted. Fig. 2. The electrical resistivity p as a function of the temperature T for two samples of semiconducting RuzGeg . Sample 1 was synthesized with ruthenium of 99.9% purity and sample 2 was synthesized with ruthenium of 99.988% purity.
corresponds, assuming free-electron behaviour, to II = 7.5 X 101* electrons cme3. However, this value has to be considered as an upper limit since the presence of a parasitic metallic phase below the 1% level could contribute a sizeable fraction of the linear term. With this concentration the degeneracy temperature is 160 K, again to be considered as an upper bound. Hall effect measurements on the same RuzGe3 sample established n-type behaviour with a Hall coefficient, expressed cubic centimeties per ampere per second, of -8.62 at 1.5 K, -7.84 at 4.2 K, -7.14 at 41 K and -3.63 at 175 K. Under the assumption of no hole contribution these values would correspond to conduction electron concentrations of 0.7 X 1018, 0.8 X 1018, 0.9 X 1018 and 1.7 X 101* cme3 respectively. The variation of the magnetic susceptibility with temperature, measured in a field of a few kilogauss, is illustrated in Fig. 3. It is apparent that RuzGe3, as well as Russia (not shown) and RuaSn3, is strongly diamagnetic. The susceptibility of semiconductors is usually given by the sum of a diamagnetic “lattice” contribution, a paramagnetic charge carrier contribution and finally a trapped electron contribution at impurities. The trapped electron contribution is visible for Ru,Ge3 below about 50 K. In the present case standard analysis yields a neutral donor concentration nd (T = 0 K) of
P5
5 .7 X lo1 * cme3, decreasing with increasing temperature and vanishing near 50 K. In order to make sure that this interpretation is correct, we prepared an RuzGe3 specimen with specially purified ruthenium (99.988%). As expected and shown in Fig. 3 the trapped electron contribution for this specimen is much smaller, corresponding to an nd (2’ = 0 K) value of approximately 1.2 X 101* cmm3. At the same time the resistivity follows the intrinsic curve to lower temperatures as indicated in Fig. 2. The almost temperature-independent diamagnetism of Ru,Ge3 in the LT state is surprisingly large, namely -0.43 X 10e6 emu g-l or -36 X low6 emu (g atom)- * . Its temperature independence over a wide range, where according to the resistivity measurements the carrier concentration varies markedly, precludes an interpretation in term8 of small effective masses (Landau-Peierls susceptibility). We believe that this strong diamagnetism can probably be explained by an important electron transfer from the germanium to the ruthenium sites, in qualitative agreement with the views expressed in Section 1. The onset of a paramagnetic contribution near 900 K in RuzGe3 is clearly related to the transition into the HT state. In order to understand this kink the carrier concentration near the upper limit of stability of the LT state, e.g. at 800 K, should be calculated. Using the standard expression and the gap from the resistivity measurement a value of 3.0 X 101* cmw3, which has to be multiplied by (m */m)3/2, is obtained. Under these conditions the compound is nondegenerate. Its Landau-Peierls susceptibility is much smaller than the temperature-independent contribution found experimentally. However, since we know that the energy gap is decreased in the HT state, a more important carrier contribution could set in once the transition is completed. The calculation shows that, in order to obtain an acceptable fit of the measurements, we would have to postulate a heavy effective mass m*/m of the order of about 6. An alternative explanation of the susceptibility in the HT state, not requiring a large effective mass, might be a gradually diminished electron transfer above the phase transition. The apparent paramagnetic contribution would then reflect instead a reduction of the “enhanced” diamagnetism.
3.3. Ru,Sn3 It can be seen from the resistivity versus temperature measurement represented in Fig. 4 that RuzSn3 is much more conducting than RusSi and Ru,Ge3. RuzSn3 clearly shows metallic behaviour at high temperatures, whereas it is probably semiconducting at lower temperatures with a very small gap. The switch-over occurs in the temperature interval between 300 and 700 K and is associated with a particularly large hysteresis. A similar effect can also be seen in the temperature dependence of the susceptibility represented in Fig. 3. These observations therefore confirm the existence of a structural phase transformation and suggest that this transformation is first order.
P6
I 0
c
I
mo
<
I
400
5
I.
600
I.
600
I.
KKJO
,J (200
T( K)
Fig. 3. The magnetic susceptibility x us. temperature T for the compounds Ru~Ge+_&, (x = t&0.25,3). Samples 1 and 2 for x = 0 are those of different purity shown in Fig. 2. Fig. 4. The electrical resistivity p us. temperature hysteresis effect should be noted.
T for metallic RuzSna.
The large
4. Conclusions It has been shown from the present analysis that the compounds RuaSis and RuzGea are semiconducting in both their HT and LT modifications. This confirms an earlier suggestion [ 5] that the electronic structure of compounds belonging to the defect TiSia structural class can be described in terms of a rigid band model. These compounds have the general composition TnB2,, (T = transition element; B z Si, Ge, Sn, Ga; n and m are integers) and are characterized by a transition metal host structure in which the B atoms are inserted in various ways and in different concentrations [6, lo] , Interestingly enough, despite the different geometry of the B-atom substructure in all these compounds the arrangement of the T atoms is roughly the same, corresponding to that of the tin atoms in p-Sn. Thus from a rigid band model point of view the B atoms are electron donors which transfer their valence electrons towards the T atoms, thereby filling transition metal d and p energy levels. Compounds such as RuAl,, Mn,,Si,, and those studied in this work (RusSis and Ru,Gea) all have 14 valence electrons per T atom site. This is exactly the number required to fill aR the d bands and the four valence bands such as those occupied in &Sn*. Since these com-
P7
pounds show a semiconducting behaviour it can be concluded that the filled bands and the unfilled bands are separated by a forbidden energy gap. In other words the electronic properties of the defect TiSia-type compounds depend essentially on the valency and the concentration of the M “filler” atoms and less on the actual position of the M atoms in the T atom host structure. With respect to Ru2Si3 and Ru,Ges such a description correlates well with the fact that the forbidden energy gap changes only slightly as the compounds undergo a first order structural phase transition. In fact this transition is known to affect mainly the positions of the silicon and germanium atoms and less the ruthenium atom host structure [ 21. Another important feature of the semiconductors Ru,Sia and Ru,Ge3 is their large diamagnetic susceptibility which appears to reflect an unusually large electron transfer to the ruthenium site. We notice that CrSip, with x a -18 X low6 emu (g atom)-’ [ 121, also shows a rather enhanced diamagnetism. With respect to RusSns, however, it becomes clear that a rigid band model can no longer be used to describe the electronic properties. In fact it can be seen from Fig. 5 that the forbidden energy gap decreases continuously in the sequence RuaSis, RuaGes, RuaSns, and Ru,Sns is already metallic. This suggests that, in TnBzn+ compounds containing heavy B elements, some of the valence levels of the B elements can cross the Fermi level, thus
RupSi3
RupGe3
Ru2Sn3
Fig. 5. The continuous variations of the forbidden energy gap E,, the magnetic susceptibility X, the unit cell volume V, the Vickers hardness Hv and the formation temperature Tf in the solid solutions Rug(B,B’)a (B,B’ = Si, Ge, Sn).
*CrSip is also a semiconductor [ 111 and has 14 valence electrons per T atom site. However, the T atom partial structure is not of the &Sn type. Thus CrSip cannot be cited as an example in favour of a rigid band model for the family of defect TiSi2 structures.
P8
reducing the amount of charge transfer from the B atom sites to the T atom sites.
Acknowledgments Stimulating discussions with Professor H. Schmid are gratefully acknowledged. We also thank Drs. A. Junod, A. Treyvaud and M. Christen and Mr. M. Pehzzone for their help with the measurements and Mr. A. Naula for technical assistance. This work was partly supported by the Fonds National Suisse de la Recherche Scientifique.
References 1 D. J. Poutcharovsky and E. Parthe, Acta Crystullogr., Sect. B, 30 (1974) 2692. 2 D. J. Poutcharovsky, K. Yvon and E. Parthe, J. Less-Common Met., 40 (1975) 139. 3 H. Nowotny, in L. R. Eyring and M. O’Keefe (eds.), The Chemistry of Extended Defects in Non-Metallic Solids, North-Holland, Amsterdam, 1970, p. 223. 4 W. B. Pearson,Acta Crystallogr., Sect. B, 26 (1970) 1044. 5 W. Jeitsehko,Acta Crystallogr., Sect. B, 33 (1977) 2347. 6 E. Parthe, in B. C. Giessen (ed.), Developments in the Structural Chemistry of Alloy Phases, Plenum, New York, 1969. 7 G. Zwilling and H. Nowotny, Monatsh. Chem., 105 (1974) 666. 8 W. Jeitschko and E. Parthe,Acta Crystallogr., 22 (1967) 417. 9 C. P. Susz, Doctoral Thesis No. 1758, Universite de Geneve, Switzerland, 1976. 10 H. Nowotny, Kristallchemie Zntermetallischer Phasen, VEB, Leipzig, 1976, and references cited therein. 11 I. Nishida,J. Mater. Sci., 7 (1972) 1119. 12 D. Shinoda and S. Asanabe, J. Phys. Sot. Jpn, 21 (1966) 555.
Pl
DIFFUSIONLESS PHASE TRANSFORMATIONS OF RuaSis, RuzGes AND RuaSns II: ELECTRICAL AND MAGNETIC PROPERTIES C. P. SUSZ* and J. MULLER Dkpartement de Physique de la Mat&e CH-1211 Geneva 11 (Switzerland)
Condense’e,
Universite’ de Gene’ve, 32 Bd. d’Yvoy,
K. YVON and E. PARTHi Laboratoire de Cristallographie aux RayonsX, CH-121 I Geneva 4 (Switzerland)
Universite
de Gene’ve, 24 Quai E. Ansermet,
(Received May 25,1979)
Summary The series of RusGes,Sn, and RusGes,Si, compounds (0 < 3c,y d 3) were investigated using ele@rical resistivity and magnetic susceptibility measurements. Except for the tin-rich compounds (x > 2) which are metallic, they are semiconducting of type n and are diamagnetic. The forbidden energy gaps for RusSis and RusGes, as measured in the high temperature modifications, are 0.44 and 0.34 eV respectively. The gap’increases discontinuously for all compounds while they undergo a first order phase transformation into the low temperature modification.
1. Introduction In two previous studies [ 1, 21 the crystal structures of RusSis, RusGes and RusSns have been examined by X-ray diffraction methods at various temperatures. The compounds, which belong to the class of so-called defect TiSis [ 31 or Nowotny chimney-ladder compounds [ 41, have been shown to undergo a diffusionless phase transformation from an orthorhombic centrosymmetric low temperature (LT) structure to a tetragonal noncentrosymmetric high temperature (HT) structure. According to X-ray measurements the temperatures at which the transformations occur decrease from about 1000 “c for RusSis to about 500 “Cfor RusGes and to below 0 C for RusSna. They are reversible and result from small displacements of the silicon, germanium and tin atoms with respect to the quasi-rigid array of ruthenium atoms forming a p-&r type of structure. Surprisingly the atomic displace*Present address: Usine genevoise de d6grossissage d’or, 4 Place des Volontaires, CH-1211 Geneva 11, Switzerland.
P2
ments are found to occur gradualIy over a wide temperature range, thus suggesting that the phase transformation is of second order. Because of the limited resolution of the X-ray powder data, however, no definite conclusion as to the absence of a small discontinuous change in the crystal structure during the transformation could be made. The purpose of the present study was to investigate the nature of this phase transition in more detail and to determine the electronic and magnetic properties of the binary compounds and their solid solutions. Of particular interest was the question of whether or not some of them show a semiconducting behaviour. In fact such a behaviour can be anticipated from a rigid band model which has been recently proposed by Jeitschko [ 5 J . This model, which is equivalent to the concept of a “partial valence electron concentration” as formulated by ParthQ [ 61, suggests that the electronic band struttures of all defect TiSis-related compounds are similar with respect to the transition metal (T) states. In particular, because of the existence of a forbidden energy gap, no more than.14 spin states per T atom site can be filled by a charge transfer from the silicon atom sites to the T atom sites. On the experimental side only a few compounds having the “magic” number of 14 valence electrons per T atom site have been studied, among which RuA12 (see ref. 5) and. Mn2,Si4, [ 71 are indeed semiconductors. However, it has been pointed out [ 83 that no compounds belonging to this st;ructural class have been found for which the total number VEC, of valence electrons per T atom site exceeds 14 by more than a few per cent (for Rhz7Ge2a VECT is 14.2). 2, Experimental The RusSia and RuaGes sainples were prepared by a two-step procedure. SmaIl pieces of ruthenium metal (99.9% purity) and germanium (99.988%) or silicon (99.99%) were melted together by inductive heating under vacuum in a water-cooled silver crucible. The samples were then remelted several times in an argon-filled arc furnace and annealed for 10 h at 1300 “c in a high vacuum furnace. Me~o~aphi~ ~sp~tion and X-ray powder diffraction analysis showed that the samples contained only a single phase. The tincontaining samples, on the contrary, had to be prepared in a different manner because Ru,Sna was found to form by a peritectoidic reaction [ 91. Well-mixed samples of ruthenium sponge and tin powder were heated in evacuated and sealed quartz tubes for as long as one month at temperatures around 1150 “C. After several cycles of grinding and annealing the resulting powder was compressed and melted in an argon-filled arc furnace. Despite several attempts to obtain a single phase the samples always contained some amount of RusSn7. The electrical resistivity measurements were performed on small bars of length 10 - 40 mm and cross section 1 mm X 1 mm. They were cut from the bulk samples by electro-erosion and were polished on all faces. For the low
P3
temperature measurements (1.2 K < T < 400 K) the electrical contacts were obtained by fixing four copper wires on these faces using a colloidal silver solution. For the high temperature measurements the contacts were made by spot welding four molybdenum wires in the case of the RupSna sample and by a simple clamping device for all other samples [ 91. This device consisted essentially of four molybdenum knife blades having a weight of 10 g each which ensured a constant pressure on the electrical contacts at all temperatures. The resistivity measurements were carried out in an induction furnace under high vacuum. In order to obtain an even temperature distribution the samples were surrounded by two concentric cylinders of tantalum foil which were closed at both ends.
3. Results and discussion 3.1. RuzSi, and RuPGea_,Si, (y = 1,2) The temperature dependence of the electrical resistivity is illustrated in Fig. 1. It can be seen that all the compounds definitely show a semiconducting behaviour and that there are some marked anomalies in the resistivity which occur in a temperature range in which structural phase transformations have previously been observed by X-ray diffraction [ 21. For instance in RuaSis the activation energy decreases very rapidly between 1100 and 1200 K, having a value of about 0.7 eV in the LT state and of about 0.4 eV in the HT state. Moreover the resistivity values obtained clearly depend on the thermal history of the sample. In fact for a given temperature the resistivity is always higher if the sample is heated up than if it is cooled down. This indicates the presence of a strong hysteresis effect which usually occurs during a first order phase transformation. When silicon is substituted by germanium the forbidden energy gap E, of the HT modification decreases. At the same time the transformation temperature to the LT modification also decreases. Interestingly enough E, is always significantly higher in the LT state than in the HT state. 3.2. RueGe, and RuzGes,Sn, (x = 0.25) It can be seen from the temperature dependence of the resistivity represented in Fig. 2 that RuzGes is semiconducting in both the LT and the HT modifications. However, the activation energy is smaller than that of the siliconcontaining compounds, being 0.34 eV in the HT state and 0.52 eV in the LT state. There is again a strong hysteresis effect between 700 and 1000 K, in accordance with the observation of a structural phase transformation in this temperature range. The semiconducting nature of RuaGe, was confirmed by a low temperature specific heat measurement. Between 1.5 and 5 K the specific heat varied as -yT + aT3, the measured Debye temperature being 435 f 10 K. The very small electronic term yielded -y = (29 f 3) X 10m6J K-’ (g atom))’ and
P4
pbO-hC?ld~
T( K)
\
‘ogP (arb.unit
5’ /
0.6
a7
0.6
103/T( K)
Cl9
10
0
2QO
400
600
6aJ
Ixx)
T( K)
Fig. 1. The logarithm of the electrical resistivity p as a function of the inverse temperature l/T for the semiconducting compounds RuzGea y Si, (y = 1, 2, 3). The hysteresis effect for RuzSia should be noted. Fig. 2. The electrical resistivity p as a function of the temperature T for two samples of semiconducting RuzGeg . Sample 1 was synthesized with ruthenium of 99.9% purity and sample 2 was synthesized with ruthenium of 99.988% purity.
corresponds, assuming free-electron behaviour, to II = 7.5 X 101* electrons cme3. However, this value has to be considered as an upper limit since the presence of a parasitic metallic phase below the 1% level could contribute a sizeable fraction of the linear term. With this concentration the degeneracy temperature is 160 K, again to be considered as an upper bound. Hall effect measurements on the same RuzGe3 sample established n-type behaviour with a Hall coefficient, expressed cubic centimeties per ampere per second, of -8.62 at 1.5 K, -7.84 at 4.2 K, -7.14 at 41 K and -3.63 at 175 K. Under the assumption of no hole contribution these values would correspond to conduction electron concentrations of 0.7 X 1018, 0.8 X 1018, 0.9 X 1018 and 1.7 X 101* cme3 respectively. The variation of the magnetic susceptibility with temperature, measured in a field of a few kilogauss, is illustrated in Fig. 3. It is apparent that RuzGe3, as well as Russia (not shown) and RuaSn3, is strongly diamagnetic. The susceptibility of semiconductors is usually given by the sum of a diamagnetic “lattice” contribution, a paramagnetic charge carrier contribution and finally a trapped electron contribution at impurities. The trapped electron contribution is visible for Ru,Ge3 below about 50 K. In the present case standard analysis yields a neutral donor concentration nd (T = 0 K) of
P5
5 .7 X lo1 * cme3, decreasing with increasing temperature and vanishing near 50 K. In order to make sure that this interpretation is correct, we prepared an RuzGe3 specimen with specially purified ruthenium (99.988%). As expected and shown in Fig. 3 the trapped electron contribution for this specimen is much smaller, corresponding to an nd (2’ = 0 K) value of approximately 1.2 X 101* cmm3. At the same time the resistivity follows the intrinsic curve to lower temperatures as indicated in Fig. 2. The almost temperature-independent diamagnetism of Ru,Ge3 in the LT state is surprisingly large, namely -0.43 X 10e6 emu g-l or -36 X low6 emu (g atom)- * . Its temperature independence over a wide range, where according to the resistivity measurements the carrier concentration varies markedly, precludes an interpretation in term8 of small effective masses (Landau-Peierls susceptibility). We believe that this strong diamagnetism can probably be explained by an important electron transfer from the germanium to the ruthenium sites, in qualitative agreement with the views expressed in Section 1. The onset of a paramagnetic contribution near 900 K in RuzGe3 is clearly related to the transition into the HT state. In order to understand this kink the carrier concentration near the upper limit of stability of the LT state, e.g. at 800 K, should be calculated. Using the standard expression and the gap from the resistivity measurement a value of 3.0 X 101* cmw3, which has to be multiplied by (m */m)3/2, is obtained. Under these conditions the compound is nondegenerate. Its Landau-Peierls susceptibility is much smaller than the temperature-independent contribution found experimentally. However, since we know that the energy gap is decreased in the HT state, a more important carrier contribution could set in once the transition is completed. The calculation shows that, in order to obtain an acceptable fit of the measurements, we would have to postulate a heavy effective mass m*/m of the order of about 6. An alternative explanation of the susceptibility in the HT state, not requiring a large effective mass, might be a gradually diminished electron transfer above the phase transition. The apparent paramagnetic contribution would then reflect instead a reduction of the “enhanced” diamagnetism.
3.3. Ru,Sn3 It can be seen from the resistivity versus temperature measurement represented in Fig. 4 that RuzSn3 is much more conducting than RusSi and Ru,Ge3. RuzSn3 clearly shows metallic behaviour at high temperatures, whereas it is probably semiconducting at lower temperatures with a very small gap. The switch-over occurs in the temperature interval between 300 and 700 K and is associated with a particularly large hysteresis. A similar effect can also be seen in the temperature dependence of the susceptibility represented in Fig. 3. These observations therefore confirm the existence of a structural phase transformation and suggest that this transformation is first order.
P6
I 0
c
I
mo
<
I
400
5
I.
600
I.
600
I.
KKJO
,J (200
T( K)
Fig. 3. The magnetic susceptibility x us. temperature T for the compounds Ru~Ge+_&, (x = t&0.25,3). Samples 1 and 2 for x = 0 are those of different purity shown in Fig. 2. Fig. 4. The electrical resistivity p us. temperature hysteresis effect should be noted.
T for metallic RuzSna.
The large
4. Conclusions It has been shown from the present analysis that the compounds RuaSis and RuzGea are semiconducting in both their HT and LT modifications. This confirms an earlier suggestion [ 5] that the electronic structure of compounds belonging to the defect TiSia structural class can be described in terms of a rigid band model. These compounds have the general composition TnB2,, (T = transition element; B z Si, Ge, Sn, Ga; n and m are integers) and are characterized by a transition metal host structure in which the B atoms are inserted in various ways and in different concentrations [6, lo] , Interestingly enough, despite the different geometry of the B-atom substructure in all these compounds the arrangement of the T atoms is roughly the same, corresponding to that of the tin atoms in p-Sn. Thus from a rigid band model point of view the B atoms are electron donors which transfer their valence electrons towards the T atoms, thereby filling transition metal d and p energy levels. Compounds such as RuAl,, Mn,,Si,, and those studied in this work (RusSis and Ru,Gea) all have 14 valence electrons per T atom site. This is exactly the number required to fill aR the d bands and the four valence bands such as those occupied in &Sn*. Since these com-
P7
pounds show a semiconducting behaviour it can be concluded that the filled bands and the unfilled bands are separated by a forbidden energy gap. In other words the electronic properties of the defect TiSia-type compounds depend essentially on the valency and the concentration of the M “filler” atoms and less on the actual position of the M atoms in the T atom host structure. With respect to Ru2Si3 and Ru,Ges such a description correlates well with the fact that the forbidden energy gap changes only slightly as the compounds undergo a first order structural phase transition. In fact this transition is known to affect mainly the positions of the silicon and germanium atoms and less the ruthenium atom host structure [ 21. Another important feature of the semiconductors Ru,Sia and Ru,Ge3 is their large diamagnetic susceptibility which appears to reflect an unusually large electron transfer to the ruthenium site. We notice that CrSip, with x a -18 X low6 emu (g atom)-’ [ 121, also shows a rather enhanced diamagnetism. With respect to RusSns, however, it becomes clear that a rigid band model can no longer be used to describe the electronic properties. In fact it can be seen from Fig. 5 that the forbidden energy gap decreases continuously in the sequence RuaSis, RuaGes, RuaSns, and Ru,Sns is already metallic. This suggests that, in TnBzn+ compounds containing heavy B elements, some of the valence levels of the B elements can cross the Fermi level, thus
RupSi3
RupGe3
Ru2Sn3
Fig. 5. The continuous variations of the forbidden energy gap E,, the magnetic susceptibility X, the unit cell volume V, the Vickers hardness Hv and the formation temperature Tf in the solid solutions Rug(B,B’)a (B,B’ = Si, Ge, Sn).
*CrSip is also a semiconductor [ 111 and has 14 valence electrons per T atom site. However, the T atom partial structure is not of the &Sn type. Thus CrSip cannot be cited as an example in favour of a rigid band model for the family of defect TiSi2 structures.
P8
reducing the amount of charge transfer from the B atom sites to the T atom sites.
Acknowledgments Stimulating discussions with Professor H. Schmid are gratefully acknowledged. We also thank Drs. A. Junod, A. Treyvaud and M. Christen and Mr. M. Pehzzone for their help with the measurements and Mr. A. Naula for technical assistance. This work was partly supported by the Fonds National Suisse de la Recherche Scientifique.
References 1 D. J. Poutcharovsky and E. Parthe, Acta Crystullogr., Sect. B, 30 (1974) 2692. 2 D. J. Poutcharovsky, K. Yvon and E. Parthe, J. Less-Common Met., 40 (1975) 139. 3 H. Nowotny, in L. R. Eyring and M. O’Keefe (eds.), The Chemistry of Extended Defects in Non-Metallic Solids, North-Holland, Amsterdam, 1970, p. 223. 4 W. B. Pearson,Acta Crystallogr., Sect. B, 26 (1970) 1044. 5 W. Jeitsehko,Acta Crystallogr., Sect. B, 33 (1977) 2347. 6 E. Parthe, in B. C. Giessen (ed.), Developments in the Structural Chemistry of Alloy Phases, Plenum, New York, 1969. 7 G. Zwilling and H. Nowotny, Monatsh. Chem., 105 (1974) 666. 8 W. Jeitschko and E. Parthe,Acta Crystallogr., 22 (1967) 417. 9 C. P. Susz, Doctoral Thesis No. 1758, Universite de Geneve, Switzerland, 1976. 10 H. Nowotny, Kristallchemie Zntermetallischer Phasen, VEB, Leipzig, 1976, and references cited therein. 11 I. Nishida,J. Mater. Sci., 7 (1972) 1119. 12 D. Shinoda and S. Asanabe, J. Phys. Sot. Jpn, 21 (1966) 555.