- Email: [email protected]
Doping studies on the origin of the band gaps in NiI2−xBrx
FIllSIgl ELSEVIER
Physica B 199&200 (1994) 637-639
Doping studies on the origin of the band gaps in NiI2_xBr J. Tang"'*, M.D. Davis a, C.J. O'Connor b, B. Wu b aDepartrnent of Physics, University of New Orleans, New Orleans, LA 70148, USA bDepartment of Chemistry, University of New Orleans, New Orleans, L A 70148, USA
Abstract The'effect of bromine doping on the magnetic properties of Nil2 was investigated. The N6el temperature TN was found 'to decrease with increasing bromine doping. This behavior can be well explained using a relationship between irN and band gap that is modified for an insulator of charge transfer type. Our study supports the suggestion that Nil2 is not a Mott insulator in the simplest sense but an insulator of charge transfer type.
There has been a renewed interest in the electronic structures and properties of the 3d transition metal compounds stimulated by the discovery of high 1", superconductivity. Whether the band gap in these materials is mainly determined by the d d Coulomb in:.'raction (U) or by the charge transfer transition from tt, e anion ,,, metal ion has been controversial [1]. According to a generalized Mott-Hubbard theory proposed by Zaanen et al. [2], for large U in the late 3d transition metal compounds the t~resence and magnitude of a band gap is essentially determined by the charge transfer e,lergy (A). The theory predicted that the band gaps in NiX2 (X = CI, Br, I) are of the charge transfer type. Optical reflectivity and photoconductivity data of the t-ansition metal halides [3, 4] were in agreement with the predictions of the theory, In this paper we repmt the results of our investigation on the magnetic properties of b:omine doped NiI2, which also indicated that the band gaps in NiI2_.~Brx are of charge transfer type. NiI2 and NiBr2 crystallize in hexagonal Cd(lz-type structure [5, 6]. They both undergo a paramagnetic to antiferromagnetic phase trensition at low temperatures.
*Corresponding author.
The antiferromagnetic state of Nil2 is characterized as helix-I type, with Nrel temperature TN of 75 K. NiBrz undergoes a paramagnetic to collinear antiferromagnetic phase transition at TN = 46 K and then enters a helix2-type antiferromagnetic state at about 23 K. Single crystal flakes of Nil2_~Br~ L: = 0.15,0.3,0.5 and 0.7) were grown using vapor condensation method in a sealed quartz tube. The mixture of NiI2 and NiBr2 was first melted at 850°C and then pulled out of the furnace at a rate of 1.5 mm/hr, and crystal flakes were obtained at the cooler end of the tube. X-ray diffraction experiments conducted on a SCINTAG diffractometer confirmed the formation of homogeneous solid solutions. Since the powders used for X-ray diffraction were flake-like and oriented preferably in the c-direction, only reflections from (0 0 1) (t = 1, 2, 3..) were significant in the X-ray patterns. Therefore, only lattice parameter c was determined for each sample, c was found to i9.63(3j, 19.54i8i, 19.5315} and 19.42[5~ for x = 0.15,0.3,0.3 and 0.7, respectively tthe c value of NiI2 is 19.808(1t ~ f-6]). The decrease of c with bromine doping reflects the reduction of the unit cell volume due to bromine substitution. The magnetic susceptibility X was measured as a function of temperature T with a SQUID magnetometer. Figure 1 shows the ~ versus T plots for the four samples.
0921-4526/94~/$07.00 '© 1994 Elsevier Science B.V. All rights reserved SSDI 0921-4526193)E0375-Q
638
J. Tang et al. / Physica B 199&200 (1994) 637-639
plotted as a function of bromine concentration for our samples (open squares). The pressure dependence study on NiIz was carried out by Pasternak and co-workers [S], and it revealed a fourfold increase in liy from 75 K at P = 0 to 310 K at P = 19 GPa. This behavior was attributed to the increased overlap between the wave functions of the 3d electrons of the cation and the 5p electrons of the anion which resulted in an increased superexchange coupling between neighboring NiZ+ moments. The band gap between the conduction and valence bands shrunk as a result of increasing bandwidth due to the wave function overlap, and as the pressure further increased the closure of the band gap was finally 0
50
loo
IS0
200
250
300
Temperature (K) Fig. I. Magnetic susceptibility x versus temperature T. (a) NiIL.88r~.I~; (W Nill,tBro.3; W NiL.&r0.5; @UNiII.&.,.
achieved. This was evidenced by the insulator to metal transition observed in Ni12 when pressure exceeded P, = 19 GPa ES]. In our study, if one were to consider only the volume effect, one would expect an enhancement in TN upon bromine doping due to the increased wave function over!ap, since the radius of Br-’ is smaller than that of !-’ and the effect of bromine substitution for iodine in NiIL is to exert a pressure on the Ni12 lattice. However, TN was found to decrease with increasing bromine doping. This controversy can be resolved when one takes into account the origin of the band gap. The relationship between Ntel temperature TN, overlap energy integral I, bandwidth B and Hubbard energy U for a simple Mott insulator was given by Anderson [9]: TN cc B’IU,
(1)
where bandwidth B = 221 and z is t?le coordination number. In Eq. (l), U is the Hubbard energy, which is a measure of the band gap for a simple Mott insulator. For an insulator of charge transfer type, the band gap is determined by charge transfer energy d, ahld U in Eq. (1) should be replaced by A, i.e., 0 Nil,
20
40
60
Composition (at. %Br)
80
100
Niir,
Fig. 2. NkeEtemperature T, as a function of bromine concentration and magnetic phase diagram of NiIz_,Br,. Open symbols are data from this study, and solid symbols are data from Ref. [IO].
The remarkable feature of these curves is that the maxima shift toward lower temperatures as the bromine doping increases. Since the maximum in the x versus T plot corresponds to the N&l temperature, which was confirmed by neutron diffraction [7], it indicates that the bromine doping tends to suppress the antiferromagnetic ordering. Shown in Fig. 2 is the NCel temperature TN
TN cc B’JA.
(2)
While rhe band gap in a simple Mott insulator U is to a large extent independent of the nature of the anion, the band gap in ap insulator of charge transfer type A is proportional to the electronegativity of the anion [4]. As bromine concentration increases, the band gap A increases as well which results in a decrease in T,,,according to Eq. (2). Thus the bromine doping has two opposite effects: one is the volume effect which tends to increase TN by incre?.sing wave function overlap; the other tends to Teduce TN by increasing the band gap through the more r- ‘. The latter is dominant in electronegative anion NiI,-.Br,. Th. observed TN reduction by bromine
J. Tang et al,/ Physica B 199&200 (1994) 637-639
doping is another evidence for the charge transfer gap in Nil2-xBrx. Room temperature resistivity p was measured for NiI2, Nill.85Bro.15 and Nill.sBro.5. Their values are 3.8×104,3.5×10 s and 5.1×105f~cm for Nil2, Nilt.85Bro.ts and Nill.sBro.5, respectively, p increases from Nil2 to Nill.sBro.5 by an order of magnitude. This is consistent with the picture that bromine doped samples have wider band gaps. Combining our data on TN and those of Moore et al. [10], we were able to construct a magnetic phase diagram for NiI2-xBrx. Figure 2 shows such a phase diagram. It will be of interest to study the phase diagram throughout the entire composition range and to study the phase boundary, if any, between helix-I and helix-2 phases in this material. In summary, the effect of bromine doping on the magnetic properties of Nil2 was investigated. The N~ei temperature TN was found to decrease with increasing bromine doping, ~'hich could only be explained on the basis that NiI2-~Brx is an insulator of charge transfer type rather than a Mort insulator in the simplest sense. Our study provides another evidence that the band gap in Nil2-xB,'~ is ef charge transfer type.
639
This study was supported through a grant from Research Corporation. References [I 2 G.A. Sawatzky and J.W. Allen, Phys. Rev. Lett. 53 (1984) 2339. [2] J. Zaanen, G.A. Sawatzky and J.W. Allen, Phys. Rev. Left. 55 (I985) 418. ['3] i. Pollini, J. Thomas and A. Lenselink, Phys. Roy. B 30 0984) 2140. ['4] C.R. Ronda, G.J. Arends and C. Hass, Phys. Rev. B 35 (1987) 4038 [5] S.R. Kuindersma, J.P. Sanchez and C. Haas, Physica B I I I (1981) 231. ['6"] A.R. West, S~lid State Chemistry and Its Applications (Wiley, New "~ork, 1984) p. 257. I 7] D. Billerey, C. Terrier, R. Mainard and A.J. Pointon, Phys. Lett. A 77 0980) 59. [8] M.P. Pasternak, R.D. Taylor, A. Chen, C. Meade, LM. Falicov, A. Giesekus, R. Jeanloz and P.Y. Yu, Phys. Rev. Lett. 65 (1990) 790. ['9] P.W. Anderson, Solid State Phys. 14 (1963) 99. ['10] M.W. Moore, P. Day, C. Wilkinson and K.R.A. Ziebeck. Solid State Commun. 53 (1985) 1009.
Physica B 199&200 (1994) 637-639
Doping studies on the origin of the band gaps in NiI2_xBr J. Tang"'*, M.D. Davis a, C.J. O'Connor b, B. Wu b aDepartrnent of Physics, University of New Orleans, New Orleans, LA 70148, USA bDepartment of Chemistry, University of New Orleans, New Orleans, L A 70148, USA
Abstract The'effect of bromine doping on the magnetic properties of Nil2 was investigated. The N6el temperature TN was found 'to decrease with increasing bromine doping. This behavior can be well explained using a relationship between irN and band gap that is modified for an insulator of charge transfer type. Our study supports the suggestion that Nil2 is not a Mott insulator in the simplest sense but an insulator of charge transfer type.
There has been a renewed interest in the electronic structures and properties of the 3d transition metal compounds stimulated by the discovery of high 1", superconductivity. Whether the band gap in these materials is mainly determined by the d d Coulomb in:.'raction (U) or by the charge transfer transition from tt, e anion ,,, metal ion has been controversial [1]. According to a generalized Mott-Hubbard theory proposed by Zaanen et al. [2], for large U in the late 3d transition metal compounds the t~resence and magnitude of a band gap is essentially determined by the charge transfer e,lergy (A). The theory predicted that the band gaps in NiX2 (X = CI, Br, I) are of the charge transfer type. Optical reflectivity and photoconductivity data of the t-ansition metal halides [3, 4] were in agreement with the predictions of the theory, In this paper we repmt the results of our investigation on the magnetic properties of b:omine doped NiI2, which also indicated that the band gaps in NiI2_.~Brx are of charge transfer type. NiI2 and NiBr2 crystallize in hexagonal Cd(lz-type structure [5, 6]. They both undergo a paramagnetic to antiferromagnetic phase trensition at low temperatures.
*Corresponding author.
The antiferromagnetic state of Nil2 is characterized as helix-I type, with Nrel temperature TN of 75 K. NiBrz undergoes a paramagnetic to collinear antiferromagnetic phase transition at TN = 46 K and then enters a helix2-type antiferromagnetic state at about 23 K. Single crystal flakes of Nil2_~Br~ L: = 0.15,0.3,0.5 and 0.7) were grown using vapor condensation method in a sealed quartz tube. The mixture of NiI2 and NiBr2 was first melted at 850°C and then pulled out of the furnace at a rate of 1.5 mm/hr, and crystal flakes were obtained at the cooler end of the tube. X-ray diffraction experiments conducted on a SCINTAG diffractometer confirmed the formation of homogeneous solid solutions. Since the powders used for X-ray diffraction were flake-like and oriented preferably in the c-direction, only reflections from (0 0 1) (t = 1, 2, 3..) were significant in the X-ray patterns. Therefore, only lattice parameter c was determined for each sample, c was found to i9.63(3j, 19.54i8i, 19.5315} and 19.42[5~ for x = 0.15,0.3,0.3 and 0.7, respectively tthe c value of NiI2 is 19.808(1t ~ f-6]). The decrease of c with bromine doping reflects the reduction of the unit cell volume due to bromine substitution. The magnetic susceptibility X was measured as a function of temperature T with a SQUID magnetometer. Figure 1 shows the ~ versus T plots for the four samples.
0921-4526/94~/$07.00 '© 1994 Elsevier Science B.V. All rights reserved SSDI 0921-4526193)E0375-Q
638
J. Tang et al. / Physica B 199&200 (1994) 637-639
plotted as a function of bromine concentration for our samples (open squares). The pressure dependence study on NiIz was carried out by Pasternak and co-workers [S], and it revealed a fourfold increase in liy from 75 K at P = 0 to 310 K at P = 19 GPa. This behavior was attributed to the increased overlap between the wave functions of the 3d electrons of the cation and the 5p electrons of the anion which resulted in an increased superexchange coupling between neighboring NiZ+ moments. The band gap between the conduction and valence bands shrunk as a result of increasing bandwidth due to the wave function overlap, and as the pressure further increased the closure of the band gap was finally 0
50
loo
IS0
200
250
300
Temperature (K) Fig. I. Magnetic susceptibility x versus temperature T. (a) NiIL.88r~.I~; (W Nill,tBro.3; W NiL.&r0.5; @UNiII.&.,.
achieved. This was evidenced by the insulator to metal transition observed in Ni12 when pressure exceeded P, = 19 GPa ES]. In our study, if one were to consider only the volume effect, one would expect an enhancement in TN upon bromine doping due to the increased wave function over!ap, since the radius of Br-’ is smaller than that of !-’ and the effect of bromine substitution for iodine in NiIL is to exert a pressure on the Ni12 lattice. However, TN was found to decrease with increasing bromine doping. This controversy can be resolved when one takes into account the origin of the band gap. The relationship between Ntel temperature TN, overlap energy integral I, bandwidth B and Hubbard energy U for a simple Mott insulator was given by Anderson [9]: TN cc B’IU,
(1)
where bandwidth B = 221 and z is t?le coordination number. In Eq. (l), U is the Hubbard energy, which is a measure of the band gap for a simple Mott insulator. For an insulator of charge transfer type, the band gap is determined by charge transfer energy d, ahld U in Eq. (1) should be replaced by A, i.e., 0 Nil,
20
40
60
Composition (at. %Br)
80
100
Niir,
Fig. 2. NkeEtemperature T, as a function of bromine concentration and magnetic phase diagram of NiIz_,Br,. Open symbols are data from this study, and solid symbols are data from Ref. [IO].
The remarkable feature of these curves is that the maxima shift toward lower temperatures as the bromine doping increases. Since the maximum in the x versus T plot corresponds to the N&l temperature, which was confirmed by neutron diffraction [7], it indicates that the bromine doping tends to suppress the antiferromagnetic ordering. Shown in Fig. 2 is the NCel temperature TN
TN cc B’JA.
(2)
While rhe band gap in a simple Mott insulator U is to a large extent independent of the nature of the anion, the band gap in ap insulator of charge transfer type A is proportional to the electronegativity of the anion [4]. As bromine concentration increases, the band gap A increases as well which results in a decrease in T,,,according to Eq. (2). Thus the bromine doping has two opposite effects: one is the volume effect which tends to increase TN by incre?.sing wave function overlap; the other tends to Teduce TN by increasing the band gap through the more r- ‘. The latter is dominant in electronegative anion NiI,-.Br,. Th. observed TN reduction by bromine
J. Tang et al,/ Physica B 199&200 (1994) 637-639
doping is another evidence for the charge transfer gap in Nil2-xBrx. Room temperature resistivity p was measured for NiI2, Nill.85Bro.15 and Nill.sBro.5. Their values are 3.8×104,3.5×10 s and 5.1×105f~cm for Nil2, Nilt.85Bro.ts and Nill.sBro.5, respectively, p increases from Nil2 to Nill.sBro.5 by an order of magnitude. This is consistent with the picture that bromine doped samples have wider band gaps. Combining our data on TN and those of Moore et al. [10], we were able to construct a magnetic phase diagram for NiI2-xBrx. Figure 2 shows such a phase diagram. It will be of interest to study the phase diagram throughout the entire composition range and to study the phase boundary, if any, between helix-I and helix-2 phases in this material. In summary, the effect of bromine doping on the magnetic properties of Nil2 was investigated. The N~ei temperature TN was found to decrease with increasing bromine doping, ~'hich could only be explained on the basis that NiI2-~Brx is an insulator of charge transfer type rather than a Mort insulator in the simplest sense. Our study provides another evidence that the band gap in Nil2-xB,'~ is ef charge transfer type.
639
This study was supported through a grant from Research Corporation. References [I 2 G.A. Sawatzky and J.W. Allen, Phys. Rev. Lett. 53 (1984) 2339. [2] J. Zaanen, G.A. Sawatzky and J.W. Allen, Phys. Rev. Left. 55 (I985) 418. ['3] i. Pollini, J. Thomas and A. Lenselink, Phys. Roy. B 30 0984) 2140. ['4] C.R. Ronda, G.J. Arends and C. Hass, Phys. Rev. B 35 (1987) 4038 [5] S.R. Kuindersma, J.P. Sanchez and C. Haas, Physica B I I I (1981) 231. ['6"] A.R. West, S~lid State Chemistry and Its Applications (Wiley, New "~ork, 1984) p. 257. I 7] D. Billerey, C. Terrier, R. Mainard and A.J. Pointon, Phys. Lett. A 77 0980) 59. [8] M.P. Pasternak, R.D. Taylor, A. Chen, C. Meade, LM. Falicov, A. Giesekus, R. Jeanloz and P.Y. Yu, Phys. Rev. Lett. 65 (1990) 790. ['9] P.W. Anderson, Solid State Phys. 14 (1963) 99. ['10] M.W. Moore, P. Day, C. Wilkinson and K.R.A. Ziebeck. Solid State Commun. 53 (1985) 1009.