Valence-level structure of phosphides of 3d-metals on the basis of XRS and ESCA data

Journal of Electron Spectroscopy and Related Phenomena, 16 (1979) 441-453 0 Elsevler Sclentlflc Publrshmg Company, Amsterdam - Prmted m The Netherlands

VALENCELEVEL STRUCTURE OF PHOSPHIDES THE BASIS OF XRS AND ESCA DATA

E P DOMASHEVSKAYA,

Department

of Physics,

V I NEFEDOV,

Instrtute (USSR)

V A, TEREKHOV

Voronezh

State Untverslty,

N P SERGUSHIN

for General and lnorganw

and YA

OF 3dMETALS

ON

A UGAI

Voronezh

(US S R )

and M N FIRSOV

Chemzstry,

US S R Academy

of Sciences,

Moscow

ABSTRACT For the fn-st time, comprehensive X-ray spectroscopic and X-ray photoelectron data have been obtained on the energy spectra of valence electrons m phosphldes of the tran&Ion metal series TIP, CrP, MnP, FeP, NIP Analysis of the experimental data on the electronrc structure of phosphldes of 3dmetals m the TIP-NIP series mdlcates that the mechamsm of mteractlon of the M 3dstates occurring near the top of the valence band with the s,p-states of phosphorus m the followmg sub-bands resides m the latter being induced near the d-sub-band as a result of the M 3d-P 3s,p mteractlon with the density maxlmum of the nearest P 3p-states being oppositely shifted as the number of d-electrons of the metal Increases Analysis of X-ray spectroscopic and X-ray photoelectron data on phosphldes of transltlon metals along with those of metals of groups I(Cu, Ag) and II(Zn, Cd), and compartson of these data with the results of sim&ir studies mvolvmg sulphldes and slhcJdes of the same metals mdlcate that the occupancy and the associated posltlon of the d-shell of the metal wlthm the valence band are the determmmg factors as far as the mutual arrangement of energy subbands and the symmetry of respective states are concerned

INTRODUCTION

Owmg to the relatively low bmdmg energy, d-electrons of transltlon metals essentially affect the properties of crystals. The overlappmg and hybndlzatlon of d- and s,p-states1-6 are responnble for the pronounced effect of d-electrons on the band structure1-9, and hence, on the optlcal properties lV6, type of conduction’ and the value of nonlmear optical susceptlblllty’* 9 The effect of d-electrons ISpartlcular~y manrfest on the latter value which undergoes sign reversal m hahdes of noble metals as opposed to all other slmllar crystals not contammg d-elements In then work’ on the dlspercqT__ of lmear susceptlblllty, Wemple and Dldomenlco decided to

442

include all d-electrons 1n evaluatmg the effective number of valence electrons 1n transition and noble metals The particular electrophyslcal properties of transition metal compounds are also determined by the presence of a vacant d-shell However, despite the great interest to theoretlclans and expenmenters of these properties and the ample literature on the subJect, the problem of interaction between d-states and other states 1n compounds has not yet been finally solved The greatest dlff1cultles are encountered by theoreticians pnmarlly because the high senslt1v1ty of d-states to the selection of a potential does not make 1t possible, with the currently exrstmg models, to derwe a theoretical structure of the energy spectra of valence electrons, that would be 1n adequate agreement with the experimental data In addition, semous calculation dlfflcultles stem from the great number of valence d-electrons 1n transition and noble metals At the same time, using such highly developed and widely applied calculation methods as OPW and APW, 1t 1s difficult to take d-states mto account This explams why 1t still remams important to study expenmentally the regulantles of the interaction between an uncompleted d-shell and other states when compounds or alloys are produced, particularly when compared with sn-nllarregulaM1es established for metals of groups I and II with filled d-shells The best approach to reveal such regulmtles 1s expenmentatlon applymg the XRS and ESCA techniques to examine vanatlons 1n the electronic structure of the valence band, caused by different degrees of f1llmg of the d-shell, as well as by the different content of transition metals 1n a compound or an alloy Despite the large amount of expenmental work of this kmd, more often than not the overwhelmmg maJonty of expenments suffer from the followmg drawbacks the lack of accurate f1t of the X-ray spectrum of a metal and a non-transltlon element to the X-ray photoelectron data, and the lack of a single approach to exammmg the electronic structure of the valence band of transition metal compounds from the pomt of view of the two basic experimental techniques Nevertheless, X-ray spectra of slllcldesl FM, phosphldesl’* l8 and sulph1des’g--24 of tram&on metals and their alloys with sillcon and alum1n1um25 reveal certam regulanties 1n the electronic structure The least known compounds 1n this group are phosph1des Until recently, only spectra of phosphorus 1n titanium and manganese phosphldes”* l8 have been avalable, which 1s far from sufficient to establish regularities 1n their electronic structure The present work 1s amed at studying the regulantles of the effect of occupancy of the d-shell of transition metals on the electronic structure of the valence band, 1n the selclesof phosphldes of transltlon metals from T1 to N1 and comparmg them with the correspondmg regular&es of this structure 1n phosphldes of noble metals of group I with a filled d-shell, as well as 1n phosphldes of metals of group II To this end, we have obtamed comprehensive X-ray and X-ray photoelectron spectra for phosphldes TIP, TIP,, CrP,

443

MnP, FeP and N1P, which have been analyzed m compmson with slmxlar data on phosphldes of metals of groups I and II, obtamed earller26827

EXPERIMENTAL

The X-ray photoelectron spectra were obtamed on an IEE-15 Vanan spectrometer, the P L,, 3 spectra were recorded with an RMS-500 spectrometer havmg a 2 m grating, the &-spectra of transltlon metals were recorded with the same mstrument having a 6 m grating, and the P &-spectra were recorded with a DRS spectrograph All spectra were obtamed using the methods described elsewhere 26*27

SPECTRA

INTERPRETATION

Figure 1 represents X-ray spectra of phosphorus (K and L) and metals, as well as X-ray photoelectron spectra of the valence states of some phosphldes of transltlon metals, matched with respect to an energy scale taken relative to the poskon of the P 2p-level (Table 1) The Kp and L,, J-spectra of phosphorus were matched with the ad of the K a 1,2 bands whose respective value A

112

I I6

A’

120

C

124 E,eV

D

128

132

II2

I I6

120

I24

128

132

E,ev

Figure 1 The X-ray and X-ray photoelectron spectra of phosphldes of transltlon 3d(--X-ray spectra of phosphorus metals X-ray photoelectron spectra (- --), ). ML= - -) [ 17 ] The energy scale IS relatxve to the P 2p-level (- - -), ““Sr,,(-

-..

~~~

TlP CrP MilP FeP NIP

T92

A’

1205 1204

A

1175 1174 1177 1179 1183 1180

1235 1234 1234 1231 1245 -

C 1255 125 5 1261 1243

B

Compound MaxtmatnP &,3-arpectrum

-.-

1274 1276 128 0 126 5 1279 1279

D 1292 1287 1293 1297

D’ 21384 21379 21379 21381 21378

B -

21355 21356 21348 21354 21352

B’ C 21348 21418 21414 21414

D

Muxama tnP Kt-spectrum

134 5 1335 1340 134 8 134 7 134 7

P2P

459 6 4595 5790 647 2 7124 858 5

M2p

energtes(eV)

Levelbtndtng

64 68

M3d 154 150 150 16 0 16 5 17 3

Valence bandwidth

OF TRANSITION : ENERGYPOSITION OF CHARACTERISTIC POINTSIN P L2,3- AND P $-SPECTRAOF PHOSPHIDES METALS ANDLEVELBINDlNG ENERGIES

TABLE1

446

of 2013 5 eV was practically mvanable wlthm + 0 1 eV. Dots m Fig. 1 delmeate that portlon of the P Kg-spectrum m FeP, supenmposed on which are the Kp 1,2 bands m the thnd order of refkctlon Spectra of different components were matched with the X-ray photoelectron data for core levels (Table 1) to wlthm f 0 5 eV We matched the P L2,3 and M L&-spectra m MnP with respect to the average value of the posltlon of the maxnnum m the M L,-spectrum relative to the P 2p-level m FeP and CrP because of the madequately reliable measurements of the bmdmg energy of the Mn 2p-level of powdered MnP Phosphldes CrP, FeP and NIP were obtamed m the form of macrocrystallme ingots synthesized directly, with an excess of the metallrc component, from a melt The single crystals of TIP and monocrystallme “needles” of TIPz were obtamed by way of two-temperature synthesis at a phosphorus overpressure28 The positron of the maxuna of the L, -spectra m all phosphldes remamed practically the same as compared with the pure metal. Tl K,, m TIP and MnKp, m MnP are taken from the literature” Therewith, the M K,,- and M Ka,,2 -spectra were matched without taking mto account the possible shift of the M K,, , 2-bands The X-ray spectra are interpreted usmg the dipole selection rule approxlmatlon with the followmg deslgnatlon of the maxnna m Fig 1 and Table 1 Maximum A corresponds to the maximum of the density of the P 3s states, while maxnnum A’ 1s connected with the P 3s-P 3s mteractlon2’~ 29 Maxlmum B corresponds to that of the den&y of the P 3p states, maximum D corresponds to that of the density of the M 3d states, and maximum C corresponds to that of the density of the M s(p) states The energy posltlons of the mam maxuna of the X-ray spectra are gwen m Table 1.

DISCUSSION

Analysis of the obtamed expenmental data @ves an mslght mto certam general regulmtles of formation of the electronic structure of valence electrons of tranatlon metal phosphrdes The lowest-energy part of the valence band (on the bmdmg energy scale) 1s occupied, mamly, by the d-states of the metal (the mam maxnnum D of the M L,-spectra) with added P 3s- and P 3p-states (pomt D) Therewlth, as the number of d-electrons m the metal mcreases, starting with CrP, an additional maximum D’ occurs m this part of the valence band, which seems to be due to the structure of the d-band itself Below the d-sub-band he two bands mcludmg, mamly, the P 3p-states (maxnnum B of the P Kp- and M Kp-spectra) and the M s@)-states (maxnnum C) with added M d-states m the first band Reliable X-ray photoelectron spectra of valence states could be obtamed only for TIP, FeP and NIP It should be noted that the X-ray photoelectron spectra of the former two phosphldes clearly reveal P 3p-states along with

446

M 3d-states At the same tune, the X-ray photoelectron spectrum of NlP reveals only d-states and almost completely comcrdes with the NI Ldl-spectrum representative of the same valence band states. The lowest sub-band A IS separated from the upper ones by an energy mterval of about 4-6 eV and represents practically pure P 3s states (mam maximum of the P Lz, 3-spectrum) with a small amount of p-states of phosphorus and the metal. Compmson of the transltlon metal phosphldes represented m Fig. 1 suggests the followmg drstmgulshmg features of vanatlons m the electromc structure of their valence band with mcreasmg number of the transltlon metal m the selcles (1) The contnbutlon of the P 3s-states to the formation of the three upper bands vmes substantially as we proceed from TIP to NIP. If the relative mten&y of a respectnfe portion of the L 2,3-spectrum (maxnna D, B and C) m TIP IS the lowest as compared to all known phosphldes of vmous types, m NIP it becomes comparable to the correspondmg mtenslty of L,, 3-spectra m phosphldes of metals of groups I and II” (11) The maxnnum of the density of p-states, as well as M s-states (pomts B and C, respectively) recedes from the top of the valence band, whereby dlstance B-D mcreases from about 2 eV m TIP to about 4 eV m NIP Note that m pure d-metals the maxnna of the L,- and Kps -spectra comclde3” The M-P bond seems to be determined, mamly, by p-type states which are densest m the C-B region, while the states concentrated near the top of the valence band and responsible for the metallic conduction of tram&on metal phosphrdes are predomxnantly of the d-type Interestmgly, as distance D-B m the TS-NIP senes increases as a result of the mam maxnnum B of the P &-spectrum shifting Just as m the M K p5 -spectrum, maximum D becomes manifest and more mtenslve as a result of the M 3d + P 3p mteractlon Thus, analysis of the expenmental data on the electromc structure of the valence band of phosphldes of d-metals m the TS-NIP senes suggests that the mechanism of interaction of the d-states of transltlon metals near the top of the valence band with the s,p-states of phosphorus m the followmg subbands resides m “pullmg” or, to be more precise, mducmg the latter near the d-sub-band as a result of the M 3d + P 3s,p mteractlon with a slmultaneous opposite shift of the maximum of the density of the P 3p&ates, which becomes more pronounced as the number of d-electrons of the metal mcreases The tendency for an increase of the contrlbutlon of the s,p-states of the non-tranetlon element of a compound to the regon near the top of the valence band with increasing number of the transltlon metal m the semes 1s also manifest m the spectra of slllcldes14, whereas m the spectra of sulphldes of transition metals it is less pronounced20-23 This tendency 1s mdlcatlve of the stronger mteractlon M 3d + A 3s,p (A - non-tram&on element) as the number of d-electrons m the metal

447

increases However, as long as the d-shell of the metal remams uncompleted, the M 3d-states are predommant m sub-band D near the top of the valence band, all the way to N1P In the only dlphosphlde - TrP, - the alternation of the sub-band, typical of monophosphldes, holds m general However, the changmg coordmatlon and, particularly, the appearance of bonds P-P = 2 26 a bnng about some changes m the electron spectrum Fast of all, as a result of the P 3s-P 3s mteractlon, the P 3s-sub-band 1s splrt producing a weak maxunum A’ m the P Lz, 3 -spectrum and makmg the contnbutlon of the P 3s,p-states in the upper part of the valence band more slgnlflcant In the structure of lsomorphous monophosphldes CrP, MnP and FeP, P-P bonds are absent3’n32, and the mam maximum of the P L2, 3-spectrum 1s single At the same time, the structure of N1P represents a vanatlon of the same distorted structure of the NlAs type featurmg pmed atoms P-P spaced 2 43 ii apart, which exceeds the covalent spacmg3* Therefore, the spacmg between the sphttmg maxima A-A’ m the P L3, 3-spectrum of NlP IS about 2 4 eV due to its being inversely proportional to the length of bond P-P 29 Unfortunately, we know of no theoretical calculations of the u-on group transltlon metal phosphldes under conslderatlon, that could be compared with the expenmental data on the electronic structure of the valence band m these compounds We can only pomt out that the known calculatrons of the band spectrum m Co1 P and Nlj P using the APW method @ve the width of the occupied states m the valence band equal to about 6--8eV, which 1s almost twice as low as the value (15-17 eV) derived expenmentally for all phosphldes under mvestlgatlon Undoubtedly, the reason for such wide dlscrepancles can be found only by theoretical studies of the electromc structure of the valence band m transition metal phosphldes and by thoroughly comparmg the results with the avalable expenmental data But even now It can be assumed that a charactenstrc feature of the valence band m transltlonmetal phosphldes 1s the prevalence of 3cLstates near the Fermi level Therewith, as long as the 3d-shell of the metal (Tl to Nl) 1s not completely filled, the alternation of energy sub-bands of a particular symmetry type remams practically the same for all compounds of transltlon metals with non-transition elements In copper, the 3cLshell IS completely filled, and its bmdmg energy becomes approximately 2 5 eV greater than m the case of tltanmm35 Exammatlon of the spectra of copper phosphldes (Fig. 2) mdlcates that they differ greatly from those of t&ruum phosphldes As was shown m the literature *OS *l* 27y 36, the mteractlons Cu 3d + P 3~ and Cu 3d + S 3p result m the P K,- and S Kpspectra m copper phosphldes and sulphldes sphttmg mto two dlstmct components In this case, the mam maxunum of these bands 1s shifted far mto the low-energy re@on slmllarly to the P &-spectrum m TIP m other phosphldes and sulphldes of tram&on metals The second maxmum remams m the high-energy reglon, almost comcldmg with the Cu 3d-states Because of

112

I16

120

I24

I26

132

136

E,eV

Figure 2 The X-ray photoelectron (- -) and X-ray spectra of phosphorus ( -) cu L, (-- -) m copper phosphldes The energy scale is relatwe to the P 2p-level

and

the low accuracy (about 1 eV) of matchmg the spectra of different components it remamed unclear whether the shortwave maxunum B’ of the P Kspectrum was the result of strong “pullmg” of the P 3p-states mto the reson of the Cu 3d-states, or whether the P 3p-states go above the Cu S&states A more detEuled study, m our work26 ) of the phosphorus spectra and the L,spectra of copper m CUP, and Cu3P has shown that the maxlma of the X-ray photoelectron and Cu La-spectra, representative of the maxunum of drstnbutlon of the Cu3d-states, do not comclde with the shortwave maxlmum B’ of the P Kp-spectra, representatlve of one of the maxima of the density of the P 3p-states This mismatch manifests itself also m the structure of the Cu L,-spectra of phosphldes as a shoulder on the shortwave side, comcldmg with the posltlon of maximum B’ and bemg the result of the mteractlon P 3p + Cu 3d Besldes, as we pass from CUP, to Cusp, the sphttmg of the P Kp-band becomes more pronounced, and maxima B and B’ are spread apart by approximately 0 5 eV This fact can be explamed by the P 3p-states being “pushed apart” on either side of the maxunum of the Cu 3d-states, as the metal concentration increases Thus, as the filled 3d-states m copper recede by about 2 eV from the Fermi level at the moment when the d-states must occupy the energy mterval,

449

normally correspondmg to the maxunum of the densrty of the P3p-states (m compounds A 3 B 5, A2 B5 ), the P 3p-band ISsplit, and most of these states approach the Fernu level. Therefore, copper phosphdes near the Fermi level accommodate, mamly, the P 3p-states with added Cu 3d- and P 3s-states The revealed regulantles of the effect produced by the filled Cu 3d-shell on the structure of the upper part of the valence band can be seen m the spectra of systems Cu-Mg, Cu-Al and Cu-SI, which had been obtamed but not explamed m earlier works25’37138 In one of them38, the Cu &-spectra were not matched with the Kp-spectra of Al on a single energy scale, however, both spectra exhibit certam regulantles m the appearance of a thm structure and Its vmatlons as a function of the copper concentration vanatlons in alloys Cu--AI, slmrlar to those we found26 m system Cu-P The sphttmg of the &$-spectrum of the light component and its increase with the copper concentration m the compound are also observed m copper sulphldes CuS and Cu, S Thus, the Cu 3d-states, lust as the S&states of other transrtlon metals, produce a certam effect on the formation of the upper part of the valence band of the compound lrrespectlve of the nature of the second component The dlstmgulshmg features of the effect of the 3d-states of copper, namely, the sphttmg and “pushing apart” of the p-states as well as the s-states bemg induced by the s-shell (maxlmum D m the P La, 3 -spectrum), are due to the filled Cu 3d-states recedmg by about 2 eV and shlftmg towards the energy interval occupied, m the absence of d-states, by the p-type electrons of the second component The presence of the shortwave maximum B’ m the Cu L,-spectrum, comcldmg with the B’ sphttmg component of the P Kp-spectrum, 1s also mdlcatlve of the slgruflcant part the Cu 3d-electrons take m the mteractlon Cu 3d + P 3p(s) A somewhat different picture IS observed m AgP2 where the Ag 4d-states feature bmdmg enewes appioxnnately 1.5-b 2 eV higher than those of the Cu 3d-states 26*27 As a result, near the top of the valence band m AgP*, mainly the P 3p(s)-states are concentrated The role of the Ag dd-states bolls down to detachmg a maJor part of the P 3s,p&ates and transferrmg them into the B’ reg;lon spaced about 4-5 eV apart from the top of the valence band In this case, the mam maximum of the P 3p-states tends to occupy the posltlon charactenstlc of non-tram&on metals (about 2139 eV on the P K,scale) and, in so domg, remams shifted by I eV towards the higher eneqqes, owmg to the repulsive actlon of the Ag 4d-states Thus, owmg to the recess of the Ag 4d-states, bands B and B’ m AgP2 mterchanged their roles as opposed to the correspondmg bands m Cup2 A slmllar sltuatlon occurs m sulphldes of Cu and Ag36 As we proceed from Cu to Zn, with the 3d-states receding further mto the lower half of the valence band, the type of metal-phosphorus mteraction changes srgnlflcantly 26027 smce the Zn 3d-states get mto the reDon of maxnnum density of the P 3s~states rather than P 3p-states, as IS the case

450

with the M 3d- and Cu 3d-states. In a previous workz7, we revealed a strong Zn 3d + P 3s interaction m zmc phosphldes Of part~ular interest 16 the result of matchmg the X-ray and X-ray photoelectron spectra m Zn,P2 (Fig. 2) Accordmg to refined data, maxunum D of the Zn 3d-states (the mam maxima of the X-ray photoelectron and Zn L,-spectra comclde) corresponds to the mmlmum of the P 3s-state component split m two. This phenomenon can be explamed as follows. Wlthout the Zn 3d-states the P L2, 3 spectrum m ZnJ P2 would be slmllar to a respective spectrum m A3 B5 and A1 B5 (0.1, P), contamang a angle mam maximum of the P 3s-states m the absence of P-P bonds m the nezghbourhood of 118 eV of the 9 L2, 3-spectrum scale. But as the Zn 3d-states get precisely into this regron, they spht the P 3s~statesmto two sub-bands on either side of their maximum and, at the same tnne, mduce P 3p-states Thus, the sltuatlon observed m ZnJPz 1s similar to Cu3P, except that the behavlour of electrons of s- and d-symmetry becomes reversed The Cu 3dstates split the P 3p-states and mduce s-states, whereas the Zn 3d-states, being much lower on the energy scale, split the P 3s-states and induce p-states Summmzed m Fig 3 are the results of comparmg the comprehensive XRS

ADd

C

ADA’C

108

112

II6

B

B

120

I24

I28

132

E,eV

Figure 3 Comparison of theoretlcai calculations of the band structure3g m Zn3Pz with experrmental data

461 and ESCA data with the theoretical band structure m Zn3P2, calculated using an approximate model without takmg mto consrderatlon the d-states m the valence band3’ The best fit between expenment and theory 1s observed m the upper part of the valence band Various data mdlcate the presence of three sub-bands m this part, and the type of symmetry of the constituent states has been determined In this case, the p-states of phosphorus are predommant near the very top of the valence band, while, Judgmg from the L, -spectrum of zmc m the reDon of C, the contnbutlon of the s-states of the metal increases with the bmdrng energy. The widest dzscrepancy between the theoretlcaI band structure and the expenmental data 1s observed near the bottom of the valence band Calculations show that this reaon features a single non-degenerate sub-band (I?, -WI ) with almost pure P 3s-states However, as can be inferred from the expenmental data, as a result of a strong mteractlon Zn 3d-P 3s(p), three sub-bands appear m this regon, two

I

,

I8

I

16

I

14

.

12

I

IO

I

I

I

I

r

6

6

4

2

0

I

-E,eV

Fqgure 4 The X-ray photoelectron (-- ) and X-ray spectra of phosphorus ( -) Zn &(- -) m zmc phosphldes The energy scale IS relative to the P 2p-level

and

462

of which m&de, mamly, P 3s_states, while the third sub-band, lymg between the former two, represents a d-band with an added amount of p-states of phosphorus Thus, calculatron of the band structure3g of Zn3Pz without takmg mto conslderatlon the d-states of zmc leads to a serrous quahtatlve disagreement wrth the experunental data on the structure of the valence band near rts bottom In zmc dlphosphdes, because of the presence of bonds of two types Zn-P and P-P m theu crystal structure, the structure of this part of the valence band becomes strll more complex, which mamfests Itself m the PLz, 3 spectra2’ (Fig. 4) Thus, the results of X-ray spectroscopic and X-ray photoelectron studres of phosphldes of transrtlon metals, such as copper and zmc, and therr companson with those of similar studies mvolvmg sulphldes and slllcldes mdlcate that the degree of fling and the associated posltlon of the metal d-shell m the valence band produce a slgnlflcant effect on the mutual arrangement of the energy sub-bands and the symmetry of respective states

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