Mukerjee, A K

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Letters to the Editor 673The author is thankful to Prof. B. N. Srivastava, D.Sc., F.N.I., for his keen interest in the problem.R E F E R E N C E SBrown, C. J., 1968, Proc. Roy. Son., A302, 185.Howells, E. R., Philips, D. C., and Rogers, D., I960, Acta Cryst., 3, 21.33E L E C T R I C  A N D  T H E R M O E L E C T R IC  P R O P E R T IE S  O F  N A T U R A L  C R Y S T A L S  O F  H E M A T I T EA. K. MUKERJEED e p a r t m e n t  o p  M a g n e t is m  I n d i a n  A s s o c ia t io n  f o r  t h e  Cu l t iv a t io n  o f  S c ie n c e ,Ca l c u t t a -3 2 , I n d i a{Received December 20, lyGH)The electrical conductivity (rr) and Sccbeck voltage (with i*(vspo(;t to Pt) (d) of crystals o f a sample of naturally occuring hematite (Brazil : Fe-^ Og 95.5%, FeO .5%, SiOj, 1.7%, TiOa 1.9%, MgO .2%, A l A  .13%, S .1%, ILO .15%) were measured both along the trigonal axis and the basal plane between 100°K and 1000°K for fresh as well as heat treated samples (figures. 1 and 2). These wore found to be w*type semiconductors and the conductivity can be represented byor =  CTg exp —AEjkTwhere the symbols have their usual meaning. The values for AE and are given in table 1.Table 1. AE and (Tq in different temperature regionsCrystal samples Fresh (a) Iloat treated (b)AJS/ in Oo Temp, range AE in Temp, rangee.v. om-1 e.v. cm1) In piano 0.079 1.38 <220 0.066 0.026 <2700.109 11.5 220<T<585 0.1020.1900.0871.20270<T<440440<T<7802) 0.066 0.76 <200 0.066 0.026 <2700.095 4.80 200<T<450 0.198 8.30 270<T<7353) Along c-axis 0.074 0.145 <205 0.080 0.0105 <3100.189 11.5 205<T<640 0.223 3.63 310<T<7004) 0.090 0.251 <210 0.095 0.029 <2600.238 14.1 210<T<440 0.179 1.14 260<T<4000.439 163 440<T<625 0.343 83 400<T<650674 Letters to the EditorAn approximate order o f the values o f effective carrier concentration {n) and mobility (fi) could be obtained from the relations :0 logfl*^ (Jonker et al, 1961, Gardner 1963)e nand (T ~  ne/t whore is the number of available states. At the valuesof n and fji are given in Table 2.Table 2. n and fi at 300°KCrystalsamples1) In plane2)3) Along fj-axis 5)Fresh (a) Heat treated (b)n per c.c*. soo*"^  n per c.c. sec”3.9x1030 2 X10211.4x1021 1.1 X 10222.5x10*2 5.6 X 10-3 1.4x10-3 3 X10-*2 X1021 5 X 1020 9.3x10201.2X10227 .8X 10“ ® 5 xlO--* 3 .7x10-3  7 .8 x  lO-oFigure 1. The conductivity of Hematite. 1 and 2 are along the basal plane, 3 and 4 alongthe c-axis and (a) and (b) are the fresh and heat treated samples.Letters to the Editor 676Figaro 2. The Soobock (>fEoot of Homatito, magiiitudo only . 1 and 2 nro along tbo basal piano, 3 and 4 along tho c-axis and (a) and (b) are tho fresh and heat troatod samples.The behaviour of cr and 0 between 600''K and lOOO^ K (figures 1 and 2) is perhaps duo to change from impurity region to intriusio region, and the permanent change in tho values of cr and 0 due to heat treatment during the first cycle o f measurement is most probably duo to the impurities and imj)orfoctions present in the natural crystals. AjE? in tho intrinsic region is about 1 o.v.The major impurity Ti is perhaps responsible for the donor centres (Morin 1951, Jonker et al 1961), which is again very possibly the source of weak ferromag­netism in it below Morin transition (Mukerjee 1967a).A study of tho values of n and /i and that of chemical analysis however sug­gest the presence of other types of carriers in it.From figures 1 and 2 it is observed that there is a conspicuous change in 0 at about 250®K, and that also there is a change in the value of AE within the temperature range 200''K to 300°K. From earlier observations (Morin 1950, also see Mukerjee 1967b) with purer samples of hematite it has been observed that there is a transition in the magnetic properties at 250°K.Detailed investigations of these properties on other samples of hematite arc in progress and the results will be published soon.676 Letters to the M itorThe author is thankful to Shri A. K. Dutta for guidance and to Prof. A. Bose for his kind interest in the work. He also wishes to thank Dr. G. F. ClaringbuU, Keeper, Natural History Museum, London for kindly presenting the natural crystal of hematite.R E F E R E N C E SOardnor, R. F. G., Swoett, F. and Taniior, D. W., 1963, J. Phya. Gbem. Solida, 24,1175. Jonker, G. H, and van Hoiiten, S., 1961, Hdlhleiter problem 6,118.Morin, F. J., 1950, Phya. Rev. 78, 819-----------  1961, Phya. Rev., 88, 1005.Mukorjee, A. K., 1967a, Indian J. Phya., 41, 460.-----------  1967b, Indian J. Phya., 41, 781

Electric and Thermoelectric Properties of Natural Crystals of Hematite

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Electric and Thermoelectric Properties of Natural Crystals of Hematite

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