MÖSSBAUER EVIDENCE OF A PHASE GAP BETWEEN Fe7Se8 AND Fe3Se4

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Submitted on 1 Jan 1979

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MÖSSBAUER EVIDENCE OF A PHASE GAP BETWEEN Fe7Se8 AND Fe3Se4

T. Tsuji, A. Howe, N. Greenwood

To cite this version:

T. Tsuji, A. Howe, N. Greenwood. MÖSSBAUER EVIDENCE OF A PHASE GAP BE- TWEEN Fe7Se8 AND Fe3Se4. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-630-C2-631.

�10.1051/jphyscol:19792219�. �jpa-00218599�

JOURNAL DE PHYSIQUE Colfoque C2, supplkment au n o 3, Tome 40, mars 1979, page C2-630

MOSSBAUER EVIDENCE OF A ?HASE GAP BETWEEN Fe7Seg AND Fe3Se4 T. ~suji*, A.T. Howe and N.N. Greenwood

Department o f Inorganic and S t r u c t u r a t Chemistry, U n i v e r s i t y of k e d s , Leeds LS2 9JT, EngZmd

RBsum6.-Des spectres Gssbauer enregistres 2 haute temperature montrent que le compose Fel,ssSe*

est un melange de deux phases, etablissant ainsi l'existence d'un "gap" de composition entre Fel,5sSe2 (phase Fe7Sea) et Fel,soSe2 (phase FesSer). Des observations precedentes par RX, mesures magnetiques et de chaleur spCcifique ont BtC r6examin6e.s B la lumiere de cette decouverte.

Abstract.- High-temperature ~assbauer studies of Fel.5sSe2 have revealed a two-phase mixture at this composition, thereby establishing the presence of a phase gap between the Fei.7sSe2 (Fe7Ses)- type phase, and the Fel.50Se2 (FesSet,)-type phase. Previous X-ray, magnetic and specific heat data have been reassessed in this light.

Recently we have studied several phases in the Fe-Se system by high-temperature Mzssbauer spec- troscopy 11-31. The technique has proved to be par- ticularly versatile, and has enabled a detailed stu- dy to be made of the Fe diffusion pathways in both Fe,+,Se, 121 and Feo.ssSe/3/, as well as providing evidence relating to the disproportionation of FeSe /I/. We report in this paper Mijssbauer data for Fe1.55Se2 up to 473 K. By making i n s i t u high tem- perature measurements we hoped to avoid, firstly, any spectral changes caused by grinding Fe-Se sam- ples 141, secondly, any effects of quenching on superlattice ordering, and finally, to ensure conti- nued sample purity with respect to oxygen.

In Fe3,xSeb the cation vacancies are ordered into every second cation layer within the NiAs-CdIz type structure, and are further ordered into strings within the layers to give a monoclinic unit cell 15-71, At higher Fe proportions X-ray measurements /5,7,8/ show an ordering change resulting in a hexa- gonal unit cell, with either 3c or 4c superlattice ordering characteristic of Fe7Ses. The different synrmetry of the vacancy ordering within the planes in Fe7Sescompared to that in FesSes would appear to preclude a continuous solid solution between the two phases. However considerable uncertainty exists re- garding the phase structure at intermediate composi- tions. Many Msssbauer studies of compositions with the nominal formula Fe7Sea 141 and Fe3Ser 191 have been reported, but data at intermediate compositions is lacking.

I . Experimental.- The sample, Fel,ssSe2, enriched x

--

Present address : Department of Nuclear Engineering, Nagoya University, Japan.

with 5 7 ~ e , was one of those used for the high-tempe- rature study of diffusional line-broadening 121, and the experimental details have been previously reported 121. The sample was sealed in a thin-walled silica cell, which was heated in an evacuated fur- nace. The oxygen content was shown to be less than 0.1 atomic X. Isomer shifts are quoted relative to Fe at 290 K.

2. Results.- Typical spectra covering the tempera- ture range 473 to 311 K are shown in the figure.

Spectra were collected within this temperature ran- ge both before and after the high temperature diffu- sion runs (870-970 K) 121, which lasted for several weeks. No changes were observed in the spectra, neither were there any differences produced by attaining a particular temperature by heating or cooling. The spectral features are therefore quite reproducible and are not dependent on thermal his- tory.

The spectra clearly show the presence of two phases with different magnetic ordering temperatu- res. Between 473 and 433 K part of the spectrum has split out into a magnetically-split resonance, which is more clearly evident at 373 and 329 K. Such an ordering temperature is consistent with those re- ported for the Fe7Ses region of 423 K 161 and 447 to 465 K (rising slightly with decreasing Fe con- tent) 171. The spectrum at 311 K shows that the central peak has now broadened due to the onset of magnetic splitting, at a temperature consistent with the NGel point of the iron-rich side of the Fe3Se+ region of 326 K /6/, or 313 K 171. The room- temperature spectrum was consistent with that of FesSer 191 after allowing for the small proportion of Fe7See. Spectra of Fel.soSe2, ofalower Fe content,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792219

showed complete spectral collapse at 329 K with no residual components at higher temperatures. The re- sults therefore show that the composition Fel.ssSe2 lies in a two-phase region between the Fe7See and Fe3 Sec regions.

Fjg. 1 : ~sssbauer spectra of Fei.ssSe2. The magne- tlcally-split Fe7Sea-type phase is evident at 433 K and below. Magnetic splitting of the Fe3Se+-type phase is seen at 311 K.

3. Discussion.- Such a two-phase region has not been previously proposed, and its existence affords a rationalisation of conflicting X-ray, specific heat and magnetic data. As 5 in Fel+,Se2 diminishes, the monoclinic distortion of the FesSer region is first evident at Fel.72Se2 /5/ or at Eel .67Se2 /7/. Howe- ver, both the observed N6el point and the associa- ted specific heat anomaly of the Fe7Se~ region per- sist up to Fe1.s9Se2/6,7/. These studies can be reinterpreted in terms of a two-phase region exten- ding from at least Fel.67Se2 to Fel.sgSe2

.

the specific heat peak decreases from a maximum at Fei.67Se2, to lower values at Fei.63Se2 and Fei .ssSe2 before disappearing at Eel. ssSe2 /7/, which is consistent with two-phase behaviour. Fur- thermore, single crystal X-ray studies showed that a sample of composition Fei.71Se2 was a two-phase mixture /8/, although little significance was atta-

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