Crystal growth and characterization of MnTe single crystals

Journal of Crystal Growth 110 (1991) 445—451 North-Holland

445

Crystal growth and characterization of MnTe single crystals 0. de Melo

*,

F. Leccabue

~

C. Pelosi

Maspec/CNR Institute, Via Chiavari 18/A, 1-43100 Parma, Italy

V. Sagredo, M. Chourio, J. Martin Department of Physics, University of Los Andes, Merida, Venezuela

G. Bocelli and G. Calestam Structural Diffractomet,y Center of CNR, 1-43100 Parma, Italy

Received 8 May 1990; manuscript received in final form 10 October 1990

The growth of MnTe single crystals by means of a chemical transport technique using iodine as a transport agent is reported. A detailed thermodynamical study of MnTe—1 2 system has been undertaken in order to define the best growth conditions. Moreover, structural, magnetic and electrical properties are reported and discussed.

1. Introduction The Mn—Te phase diagram admits only two compounds, MnTe and MnTe2 in particular, MnTe exhibits a cubic (NaC1) structure at high temperature and a hexagonal (NiAs) structure at relatively low temperature (< 10000 C). The NiAs-type compound MnTe is a typical antiferromagnetic semiconductor [1—3].Its electrical magnetic thermal and optical properties have been investigated by several authors using both polycrystalline [4—7]and single crystal [3,8,9] materials. Mateika [10] has obtained MnTe single crystal by using the travelling heater solution method with an excess of Te and a boron oxide molten layer on MnTe—Te melt to prevent the evaporation of Te. Zanmarchi [3] has used the Bridgman method but the quality of the crystals obtained

was not too good due to the presence of cavities and grain boundaries. Very few data are reported in the literature about the MnTe growth using vapour phase transport; only Sadeek in his thesis [11], has reported the growth of MnTe single crystals by chemical vapour transport technique. In the present work, we report the growth of MnTe by means of a chemical vapour transport technique (CVT) in a closed tube using ‘2 as a transport agent. A detailed thermodynamical study on the MnTe—12 system has been undertaken to define the best growth conditions; the presence of Mn12 as liquid phase during the growth process is discussed. Structural, magnetic susceptibility and resistivity measurements, and Raman spectra of MnTe single crystal are also reported.

2. Crystal growth and thermodynamical study *

**

Permanent address: Physics Faculty, La Habana Univer-

sity, Habana, Cuba. To whom correspondence should be addressed,

0022-0248/91/$03.50 © 1991

—

The CVT technique has proved to be the most suitable forpoints growing that present highmethod melting andcompounds phase transforma-

Elsevier Science Publishers B.V. (North-Holland)

446

0. de Melo et al.

/ Growth

and characterization of MnTe single crystals

tions as in the case of MnTe and in addition the crystals exhibit high purity and crystalline perfection. The polycrystalline materials and 12, the transport agent, were sealed in an evacuated (10.~6 Torr) quartz ampoule. The experimental growth conditions are reported in table 1. Fig. 1 shows some MnTe single crystals grown under the fol-

lowing conditions: average temperature TG 700°C. temperature gradient ~T 30°C, total pressure P 0.5 atm, growth time t 2 weeks, transport mass length / 90—100 mm, and quartz tube with 20 mm in diameter and 180 mm in length. A furnace with eight heating zones permitting different temperature gradient and profiles was used. =

=

=

=

=

JL lOmrn

10mm Fig. 1.. MnTe single crystals.

0. de Melo et a!.

/ Growth and characterization

The MnTe—12 system has been studied in detail and in order to predict the optimum growth conditions, pressure and mass transport,i.e.wetemperature, carried outtotal a thermodynamical investigation. We took into account the following set of reactions: MnTe(s)

+ 12

+ 12

(Tel)2

=

=

+

~ Te~,

(Tel)2,

2 Tel,

2 I, MnTe2(s)

+

Mn12(l)

Mn12(g),

12

=

=

‘2

Table 2 Thermodynarnical data(~Hf), used inentropies the present calculations: the formation enthalpies (~1Sf) and the heat 5 capacity (Cr) given by the expression: a + b x iO—~ T + c X i0 1-2 Cornpound

~lHf

z~S

(kcal/mol)

(eu.)

C,, (cal/mol)

(1)

Mn12(g)

—10.22

79.8

14.4

(2)

Mn12(l)

—5L676 —58.0

39.863 36.0

26.0 16.06

6.53

(3)

25.5

(4)

Mnl(g) MnTe(s) Tc2(g)

—29.9 40.25

22.4 64]

13.55 8.636

0.66 0.282

(5)

TeI(g) (Tel)2(g)

129 69.0

290.0 420.0

(6)

l2(g) 1(g)

b

~

Mn1

=

2 Te2

447

of Mn Te single crystals

=

Mn12

+

Te2,

together with the two following linear equations:

Mnl2(s)

MnTe2(s)

14.919

62.277

25.52

43.18

—30

34.66

47.0 88.0 8.94 4.874

0.136 0.096

—0.148 0.067

18.32

(7) ~nPMn

=

(8)

~qTePTe’

which represent Dalton’s law and the stoichiometnc constraints, respectively (qM~and ~Te are the stoichiometric coefficients of Mn- and Te-containing species). The set of thermodynamical data [12,13] used in calculating the equilibrium constants of the above reactions are reported in table 2. The growth conditions are of particular interest since the MnTe compound is limited on the Mnrich side by the presence of Mn12 (liquid or solid) and on the Te-rich side by the presence of MnTe2. The calculation results are reported in fig. 2, where the partial pressures of the gaseous species inside the growth system are reported for different concentrations of 12 transport agent (i.e., total Table I Experimental growth conditions for growing MnTe single crystals Note

pressure varying from 1 to 0.2 atm), as a function of temperature. The main result is the prediction that the growth of MnTe single crystal is possible only using low concentration of iodine, since at high concentration of iodine, Mn12 in its solid and liquid forms is the stablest species at the low temperature range. In order to avoid the presence of Mn12 liquid, a higher growth temperature (TG> 970° for total pressure of 1 atm) is needed, but at these growth temperatures MnTe rock-salt is formed. The presence of the liquid Mn12 can affect seriously the growth conditions, since both the Mn and the 12 species are captured from the vapour phase. The lack of the former can change the stoichiometry of the binary compound and strongly reduce the mass transport and therefore the growth rate. It is worth noting that the MnTe2 compound can be grown together with MnTe, in a particular condition of gaseous phase stoichiometry, namely in Te-rich conditions. Under the conditions taken into consideration in our calculations, the presence of MnTe2 was shown to be avoided.

T2

T1

P,01

(°C) 800

(°C)

(atm)

770

1

Large presence of liquid phase 2

3. Characterization

710

690

0.5

670

640

1

crystals, Dark platelets, little presence 2X 1 cmof liquid phase Few crystals

Small fragments of the large single crystals were characterized by single crystal X-ray diffrac-

448

0. de Melo et a!.

/ Growth and characterization

tion which shown a NiAs-type structure in correspondance to the low temperature form of MnTe. The lattice parameters were refined by least-square fitting of the setting angles of 25 selected high U reflections (150 < 0 < 250) utilizing graphite

of Mn Te single crystals

rnonochromatized Mo Ka radiation (A 0.71069 A) which showed no significant variation for different crystals. The lattice parameters, measured at 295 K, are a 4.147(1) A and c 6.703(1) A, in good agreement with the literature data [14]. In =

=

=

—

12 —

—

(Tel)

—2

2

_—

I 3

Tel

—

— ..-

—

—

Mn12 (liq.)

C, 0 -J

—b

Te2

—6

A) ~‘tot. 0.2 atm.

a 800

900 1000 TEMPERATURE (K)

1100

1200

12

— — — —

(Tel)2 -2 Tel

-

3

~ —

/

Mnl~(liq.)

C,

C

—~

-5 -6

T e2

8)

~

atm.

b 800

900

1000

1100

1200

TEMPERATURE (K) Fig. 2. Partial pressures resulting from the heterogeneous equilibrium MnTe—12 system as a function of temperature, when the total pressure is set at: (a) 0.2, (b) 0.5 and (c) I atm.

0. de Melo et a!.

(Iel)2

/

449

Growth and characterization of Mn Te single crystals

Mn1

—

2

—

—

j: ~ i—j:

—

Mn 12

Te2

—6

C) ~tot. 1 atm.

I

800

C

I

900

1000 TEMPERATURE (K) Fig. 2 (continued).

order to localize possible vacancies or additional atoms in the structure, which may arise by a deviation from the ideal 1: 1 stoichiometry [10], a

1100

1200

crystal structure analysis, in the space group P63/mmc, was performed. No evidence of additional atoms or vacancies in the atomic positions

2,3

2,2

-I ~

2,1

Ew

C.,

C’, Ct) 2,0

1,9 100

150

200

250

300

TEMPERATURE (K) Fig. 3. Magnetic susceptibility of MnTe single crystal as a function of temperature.

350

450

0. de Melo et a!.

/

Growth and characterization of Mn Te single crystals

0,85

0,80

E 0

01 >I~

0,75

U) U,

w

0,70

0,65 290

300

I

I

310

I

320

330

340

TEMPERATURE (K) Fig. 4. Resistivity of MnTe single crystal in the neighbourhood of the Néel temperature.

resulted from the structure refinement, so that the analyzed crystals can be considered stoichiometric on the basis of X-ray diffraction, The magnetic susceptibility was measured by Faraday method using platinum NBS as reference standard. The magnetic field was about 2700 Oe and the temperature range of measurement was between 80 and 350 K. Fig. 3 shows the magnetic susceptibility as a function of temperature. It is possible to see an increase of the susceptibility as the temperature increases from 80 K; this is a typical behaviour for an antiferromagnetic state. At 325 K a transition from the increasing behaviour is observed which represents the Néel temperature, characteristic parameter in the transition from the antiferromagnetic to the paramagnetic state. This value is slightly higher than the value reported by Kamat Dalal and Prabhu [5], but is in good agreement with that reported by Allen et al. [8]. In order to perform the resistivity measurements four gold contacts were evaporated over a clean surface of a 1 x I x 0.4 cm3 platelet MnTe single crystal. Resistivity was measured between 293 and 333 K, i.e. in the neighbourhood of the Ned temperature. Fig. 4 shows the resistivity be-

haviour as a function of temperature. A maximum in the resistivity was observed at 307 K. This value is generally correlated to the Néel temperature and it is similar to that observed by Kamat Dalal and Prabhu [5] in sintered polycrystalline samples. The resistivity values were quite different, i.e., our

A

=

~

B C

>.

~

I

E

F

—

G

90

.

~0

WAVE NUMBER

210

(cm1)

Fig. 5. Spectrum of MnTe single crystal excited with 514.5 nm Ar~laser radiation at room temperature.

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Growth and characterization of MnTe single crystals

451

single crystals show higher resistivity, greater by one order of magnitude. The different values of the maximum in the susceptibility and resistivity

nique, using ‘2 as transport agent. Because of the formation, during the growth process, of spurious species such as Mn12(I), Mn12(s) and MnTe2(s), a

curves either in this work or in the literature data could be due to anisotropy effects. The Raman spectrum was obtained using the excitation wavelength of 5145 A of an argon ion laser. The experiment was performed in the backscattering configuration. The Raman scattered radiation was analysed using a Spex-Ramalog 5 instrument with a double spectrometer and detected with a photomultiplier with the usual photon counting electronics. The interesting portion of the Raman spectrum at room temperature, recorded in the present investigation, is shown in fig. 5. We observed more peaks than that predicted by the group theory. Since MnTe crystal is described by group D~,the phonon spectrum at the Brillouin zone point can be analysed qualitatively by a decomposition into the irreducible representation:

compromise between low temperature growth and low ‘2 concentration (i.e., low total pressure) must be taken into account. The electrical and magnetic measurements show a little difference (ranging from 307 to 325 K) regarding the indication of the Néel temperature, probably due to anisotropic effects.

F=

2 A2,,

+

Big

+

B1,,

+

2 E1,,

+ E2g +

Acknowledgements Two of the authors (V.S. and M.Ch.) wish to thank CDCHT, Universidad de Los Andes, for for financial assistance trought Project C-204. One of the authors (0. de Melo) thanks the International Center for Theoretical Physics of Trieste (Italy) for a fellowship.

E2,,,

where only one E2g mode is Raman active. Four phonon modes were observed at 115, 128, 171 and 196 cm’ (peaks A, C, F and G); these are very similar to the transversal and longitudinal modes of optical phonons assigned by Allen et a!. [8], in particular for polarization E ii c, WTO 115 cm~ and ~~LO 170 cm’. The peak E might be attributed to some combination modes between 128 1 (peaks C and F). Moskovits et al.’s and results 171 cmsuggested that the peaks A and B can [15] be assigned to the transitions between diatomic manganese and free tellurium in the compound, but our growth conditions and X-ray measurements give us indications of the absence of the above species. =

=

4. Conclusion Large single crystals of NiAs-type MnTe (2 X 1 cm2) have been grown by means of CVT tech-

References [1] [2] [3] [4]

K. Sugihara, J. Phys. Chem. Solids 33 (1972) 1365. J.A. Wasscher and C. Haas, Phys. Letters 8 (1964) 302. G. Zanmarchi, J. Phys. Chem. Solids 28 (1967) 2123. Y.P. Yadava, Mater. Letters 6 (1988) 297.

[5] V.N. Kamat Dalal and RB. Prabhu, Physica B112 (1982)

42. [6] S. Onari, andPhys. K. Kubo, Solid State Commun. 14 507.T.J.Arai [7] (1974) W. Palmer, Appl. 25 (1954) 125. [8] J.W. Allen, J. Lucovsky and J.C. Mikkelsen, Jr., Solid State Commun. 24 (1977) 367. [9] K. Ozawa, S. Anzai and Y. Hamaguchi, Phys. Letters 20 (1966) 132. [10] D. Mateika, J. Crystal Growth 13/14 (1972) 698. [11] A.H.J. Sadeek, Thesis, Rensselaer Polytecnic Institute (1971). [12] I. Barin, 0. Knacke and 0. Kubaschewsky, Thermodynamical Properties of Inorganics substances, Supplement (Spnnger, Berlin, 1977). [13] G. Dittmer and U. Nieman, Philips J. Res. 37 (1982) 1. [14] 5. Greenwald, Acta Cryst. 6 (1972) 698. [15] M. Moskovits, D.P. DiLella and W. Limm, J. Chem. Phys. 80 (1984) 626.