Thermoelectric features and magnetic properties of Pr { Fe , Co , Ni - PowerPoint PPT Presentation

Hvar Workshop on Correlated Thermoelectrica, September 2005 Thermoelectric features and magnetic properties of Pr { Fe , Co , Ni } 4 Sb 12 E. Bauer Institute of Solid State Physics A - 1040 Wien, Austria 26. September 2005 in cooperation with: St. Berger, G.Hilscher, R. Lackner, H. Michor, Ch. Paul, M. Reissner, W. Steiner A. Grytsiv and P. Rogl, Vienna, Austria E. Scheidt, Augsburg, Germany A.D. Hillier, D.T. Adroja, ISIS, UK

Hvar Workshop on Correlated Thermoelectrica, September 2005 Thermoelectric features and magnetic properties of Pr { Fe , Co , Ni } 4 Sb 12 E. Bauer Institute of Solid State Physics A - 1040 Wien, Austria 26. September 2005 in cooperation with: St. Berger, G.Hilscher, R. Lackner, H. Michor, Ch. Paul, M. Reissner, W. Steiner A. Grytsiv and P. Rogl, Vienna, Austria E. Scheidt, Augsburg, Germany A.D. Hillier, D.T. Adroja, ISIS, UK

Motivation and Overview • Thermoelectric conversion: complex multinary compounds for – large values of the Seebeck coefficient S – high Peltier coefficients Π I = STI • Knowledge of principal interaction mechanisms • Requirements for thermoelectricas and scenarios for possible improvements • Dominating role of rare earth ions for ground state properties – Superconductivity – Heavy Fermion behaviour – Metal to insulator transitions – Mixed and intermediate valence

Formation of skutterudites

Formation of skutterudites • 7 subgroups • stable binaries Formation of Skutterudites EpT 4 X 12 with Co, Rh, Ir H X' X X" He and P, As, Sb Li Be B C N O F Ne Na Mg T' T T" Al Si P S Cl Ar • 72 compensated K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr electrons ⇒ dia- Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn magnetic semi- Fr Ra Ac Ku Ns conductors La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu • ternaries are Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr electronically stabilised.

Formation of skutterudites

Formation of skutterudites • 7 subgroups unfilled skutterudites binary ternary T 4 X 12 T' 4 X 12 T" 4 X 12 T 2 X' 6 X" 6 T' 2 T" 2 X 12 T' 4 X 8 X" 4 T" 4 X 8 X' 4 • stable binaries Co 4 P 12 “Fe 4 Sb 12 ” Ni 4 P 12 Co 4 Ge 6 Te 6 Fe 2 Ni 2 Sb 12 Fe 4 Sb 8 Se 4 Ni 4 P 8 Ge 4 Co 4 As 12 Pd 4 P 12 Co 4 Sn 6 Se 6 Fe 2 Ni 2 As 12 Fe 4 Sb 8 Te 4 Ni 4 Bi 8 Ge 4 with Co, Rh, Ir Co 4 Sb 12 Co 4 Sn 6 Te 6 Fe 2 Pd 2 Sb 12 Ru 4 Sb 8 Se 4 Pt 4 Sb 7.2 Sn 4.8 Rh 4 P 12 Co 4 Ge 6 S 6 Fe 2 Pt 2 Sb 12 Ru 4 Sb 8 Te 4 Ni 4 Sb 8 Sn 4 and P, As, Sb Rh 4 As 12 Co 4 Ge 6 Se 6 Ru 2 Ni 2 Sb 12 Os 4 Sb 8 Te 4 Ni 4 As 8 Ge 4 Rh 4 Sb 12 Rh 4 Ge 6 S 6 Ru 2 Pd 2 Sb 12 Ir 4 P 12 Ir 4 Ge 6 S 6 Ru 2 Pt 2 Sb 12 Ir 4 As 12 Ir 4 Ge 6 Se 6 • 72 compensated Ir 4 Sb 12 Ir 4 Sn 6 S 6 Ir 4 Sn 6 Se 6 electrons ⇒ dia- Ir 4 Sn 6 Te 6 magnetic semi- filled skutterudites ternary Quaternary EpT 4 X 12 EpT' 4 (X 1-x X' x ) 12 Ep(T 1-x T' x ) 4 X 12 conductors T = Fe, Ru, Os T' = Co, Ir T = Fe; T' = Co X = P, As, Sb X = Sb; X' =Ge, Sn X = Sb Ep =Ca, Sr, Ba, La, Ce, Pr, Nd Ep = La, Nd, Sm, Tl Ep = Tl • ternaries are Sm, Eu, Gd, Tb, Yb, Th, U LaIr 4 Sb 9 Ge 3 ,NdIr 4 Sb 9 Ge 3 , TlFeCo 3 Sb 12 LaFe 4 P 12 , UFe 4 P 12 , ThOs 4 As 12 , SmIr 4 Sb 9 Ge 3 ,TlCo 4 Sb 11 Sn, electronically YbFe 4 Sb 12 , ... La 0.9 Co 4 Sb 10.3 Sn 2.44 , …. stabilised. metastable partially filled skutterudites: Ep 1-y Fe 4 Sb 12 ; Ep = Na,Y, Hf, Sn, Lu Ep 1-y Co 4 Sb 12 ; Ep = Sn, Pb

Filled skutterudites

Filled skutterudites • structure type: LaFe 4 P 12 ( CoAs 3 -structure). • lattice parameter: c a = 9 . 127 ˚ A ( PrFe 4 Sb 12 ) • a strongly dependent on el.positive pnictogen atom (change as element z (e.g. Pr, Nd ) large as 15 % ) y x Sb, (P,As) • RE ion sixfold co-ordinated by X. d-element • extremely large atomic dis- (Fe, Co, Rh ...) b placement parameter of fil- a ler elements; increases with increasing cage volume; in- creases with decreasing io- nic size.

Pr-based skutterudites

Pr-based skutterudites • superconductivity in PrRu 4 As 12 (Shirotani et al., 1997) and PrRu 4 Sb 12 below 2.4 and 1 K (Takeda et al., 2000) ; • a metal to insulator - and structural phase transition in PrRu 4 P 12 at T MI = 60 K Sekine et al., 1997, Lee et al., 2001 , • Magnetic ordering at T N = 6 . 2 K in PrFe 4 P 12 (Torikachvili et al., 1987) . But: antiquadrupolar order parameter? (Hao et al., 2002) – Kondo-like anomalies; C p /T for T → 0 shows a huge value of about 1.4 J / molK 2 (Sato et al., Matsuda et al., 2000) . – de Haas van Alphen measurements evidenced extraordinary heavy electrons m ≈ 70 m 0 (Sugawara et al., 2001) , suggesting strongly correlated electrons. • Superconductivity and heavy fermion behaviour in PrOs 4 Sb 12 ( E.D. Bauer et al., 2001 ). T c = 1 . 8 K; Γ 3 non-magnetic ground state.

Electronic features and interactions of filled skutterudites

Electronic features and interactions of filled skutterudites • Kondo effect (Ce, Yb, Pr!); Hybridisation and band positions • RKKY interaction; in skutterudites ( Harima 2001) • crystal field splitting; – non-magnetic CEF ground state possible for non-Kramers ions, e.g. Pr; – quadrupolar moment 3 J 2 z − J ( J + 1) possible; • p − f mixing plays a crucial role • large coordination number in skutterudites

Magnetic order in Pr 0 . 73 Fe 4 Sb 12 1.2e-4 14 1.0e-4 12 1/ χ [mole/emu] 10 • Magnetic pha- 8.0e-5 8 se transition at χ a.c. [m 3 /kg] 6 T mag = 4 . 5 K; 6.0e-5 4 • Order associa- 2 ted with locali- 0 4.0e-5 0 5 10 15 20 25 30 zed Pr- 4 f elec- T [K] trons; 2.0e-5 Pr 0.73 Fe 4 Sb 12 0.0 0 5 10 15 20 25 30 T [K]

Magnetic order in Pr 0 . 73 Fe 4 Sb 12

Magnetic order in Pr 0 . 73 Fe 4 Sb 12 • no spontaneous 2.0 1.2e-4 magnetisation T = 2 K 14 → AFM?! 1.0e-4 12 1/ χ [mole/emu] 1.5 10 • Absence of 8.0e-5 8 M [ µ B/f.u.] χ a.c. [m 3 /kg] saturation in 6 1.0 6.0e-5 4 M ( H ) at 6 T 2 0 • Reduced value 4.0e-5 0 5 10 15 20 25 30 0.5 T [K] of 2.6 µ B at 6 2.0e-5 Pr 0.73 Fe 4 Sb 12 Pr 0.73 Fe 4 Sb 12 T compared to 0.0 0.0 free Pr 3+ 0 5 10 15 20 25 30 0 1 2 3 4 5 6 µ 0H [T] T [K] with gµ B J = 3 . 2 µ B

Magnetic order in Pr 0 . 73 Fe 4 Sb 12 : elastic neutron scattering (ROTAX at ISIS)

Magnetic order in Pr 0 . 73 Fe 4 Sb 12 : elastic neutron scattering (ROTAX at ISIS) • no resolveable in- tensity between low and high tempera- tures - neither at nuclear Bragg peaks (FM state) nor at additional (hkl) va- lues (AFM). • FM more likely • some corroboration by B. Maples group.

Magnetic order in Pr 0 . 73 Fe 4 Sb 12 : µ SR spectroscopy at ISIS • damping at high tem- peratures dominated 0.30 by static nuclear mo- 15K 15K 6K ments; 5K 4.6K 0.25 3.6K • decay functi- 2K 0.03K Asymmetry on: G ( t ) = 0.20 a exp( − λt ) KT ( − σ KT t ) 0.15 T = 15 K: µ s − 1 , λ = 0 . 045 0.10 0.03K σ KT = 0 . 14 µ s − 1 ZF • damping rate σ increa- 0.05 0 1 2 3 4 Tim e ( µ s) ses with decreasing T . • NO sign of frequency zero field µ SR spectra at various fluctuations due to Pr temperatures ordering!

Magnetic order in Pr 0 . 73 Fe 4 Sb 12 : µ SR spectroscopy at ISIS 2500G 0.30 • a minimum and a ma- 0.25 700G ximum evolves bet- Asymmetry 300G 100G 0.20 ween 50 and 700 G Conclusions: complex ma- 0.15 gnetic structure with re- 50G duced moments; or qua- 0.10 drupolar degrees of free- T=0.03K 0G 0.05 dom?! 0 1 2 3 4 Time ( µ s) µ SR spectra at various external fields at 0.03 K.

Paramagnetic properties of Pr 0 . 73 Fe 4 Sb 12 • Curie-Weiss behavior of 1 /χ ( T ) with 160 µ eff =4.15 µ B ; Pr 0.73 Fe 4 Sb 12 , 140 corected for Fe µ eff (Pr 3+ ) = 3 . 58 µ B 120 • small positive paramagne- 1/ χ [mol/emu] tic Curie temperature → 100 FM? 80 • Deviation from Curie- Pr 0.73 Fe 4 Sb 12 , 60 Weiss at lower tempera- raw data 40 tures ⇒ CEF effects 20 • Correction of data accor- ding to 0 χ = χ Pr + χ [Fe 4 Sb 12 ] 0 50 100 150 200 250 T [K] ⇒ isolation of χ Pr • Extrapolation to 100% Pr

Crystal electric field effects: basics y-axis R Q j energy e.g., |5/2 > Γ 7 Charge distribution R j ρ (R) ground state multiplet j = 5/2 (e.g., Ce, Sm) N = 2j+1 = 6 e.g., |3/2 > x-axis Γ 8 r i Rare earth RE 3+ e.g., |1/2 > (4f) N = (nl) N e.g., hexagonal cubic symmetry symmetry � B m l O m H CF | Ψ � = E | Ψ � with H CF = l l,m B m . . . crystal field parameters (to be determined by experiment or l theory); O m . . . Stevens Operators l H hexa = B 0 2 O 0 2 + B 0 4 0 0 4 + B 0 6 O 0 CF 6

Crystal electric field effects: basics

Crystal electric field effects: basics y-axis energy R Q j Γ 3 Charge distribution R j ρ (R) ground state multiplet Γ 4 j = 4 (e.g., Pr) N = 2j+1 = 9 x-axis Γ 1 r i Rare earth RE 3+ (4f) N = (nl) N Γ 5 e.g., cubic symmetry � B m l O m H CF | Ψ � = E | Ψ � with H CF = l l,m B m . . . crystal field parameters (to be determined by experiment or l theory); O m . . . Stevens Operators l H cub. CF = B 0 O 0 4 + 5 O 4 + B 0 O 0 6 − 21 O 6 � � � � 4 4 6 6