Steric Effects on the. Transition in YH x

Transcription

1 Steric Effects on the Metallic-Mirror Mirror to Transparent-Insulator Transition in YH x Troy C. Messina Department of Physics University of Texas at Austin Final Defense 22 November 2002

2 Outline Introduction to Switchable Mirrors Y 1-z Sc z H x Samples Film synthesis Gas Hydrogenation Optical Transmittance and Resistivity Structure XRD of as deposited and dihydride Y-Sc alloy films Results Summary

3 Introduction to Switchable Mirrors Thin films of Y and La show a quick and reversible transition from a metallic-mirror mirror to a transparent- insulator state due to hydrogen absorption Theory LDA - displacements from HoD 3 (Kelly) = small bandgap GW - HoD 3 with 3 ev bandgap; no ionic H - (van Gelderen) LDA w/ H ev gap, but nature of optical change? (Ng) Structural dependence studied in La 1-z Y z H x La remains fcc for 2 x 3 Y hcp (x < 2), fcc (x = 2), hcp (2 < x 3)

4 Introduction to Switchable Mirrors QuickTime?and a GIF decompressor are needed to see this picture. Movie created by R. Griessen et al., Vrije University, Amsterdam, Holland

5 Introduction to YH x 3 distinct phases in ρ & T α-phase Increase in ρ β-phase Decrease in ρ Peak in T at λ ~ 700 nm γ-phase phase Large increase in ρ and T

6 Introduction to ScH x nm Sc / 10 nm Pd T (820 nm) ρ (µω cm) 2 distinct phases in ρ & T α α + β β β + H 2 α-phase Increase in ρ β-phase Decrease in ρ Peak in T at λ ~ 820 nm

7 Introduction to Y 1-z Sc z H x YH x crystal structure α-phase (x x < 0.21) Hexagonal (hcp) lattice Hydrogen atoms primarily occupy tetrahedral sublattice

8 Introduction to Y 1-z Sc z H x YH x crystal structure β-phase (1.8 < x < 2.1) ScH x Cubic (fcc) lattice Hydrogen atoms primarily occupy tetrahedral sublattice crystal structure α-phase (x x < 0.21) β-phase (1.8 < x < 2.0) Cubic (fcc) lattice Hydrogen atoms primarily occupy tetrahedral sublattice Y, Sc H Tetrahedral H Octahedral

9 Introduction to Y 1-z Sc z H x YH x crystal structure γ-phase (x x > 2.8) Hexagonal (hcp) lattice, larger than α- phase Hydrogen atoms primarily occupy octahedral sublattice

10 Introduction to Y 1-z Sc z H x YH x crystal structure γ-phase (x x > 2.8) Hexagonal (hcp) lattice, larger than α- phase Hydrogen atoms primarily occupy octahedral sublattice ScH x crystal structure γ-phase does not exist!!!

11 Introduction to Y 1-z Sc z Y 1-z Sc z forms a solid-solution alloy for all z

12 Y 1-z Sc z Alloy Synthesis Arc melt stoichiometric mixtures of Y and Sc in inert atmosphere zirconium gettered chamber 100 nm films on 1 x 1 cm 2 glass and quartz substrates UHV dual electron beam evaporation 10 nm palladium cap for anti hydrogen absorption anti-oxidation and catalyzation of Calibrated crystal oscillator film deposition monitors

13 Partial pressure mass flow control of hydrogen gas to 0.1% H 2 :Ar Rotating reflection grating transmission nm ( ev) 4 point in-line resistivity Experimental V 2 I out I in V 1 l w

14 XRD of Y 1-z Sc z Alloys Sc induced reduction in lattice parameters correspond to a cell volume decrease of ~38% H incorporation (x ~ 0.3) during film growth change in lattice parameters NOTE: UNIT CELL VOLUME V cell (z = 0.20) = V cell (Lu) = 177 Å 3

15 XRD of Y 1-z Sc z H 2 Alloys Measured fcc lattice parameter, a, is within 1% of literature values for YH 2 and ScH 2 Phase separation after hydriding observed in z = 0.40 film

16 Y 1-z Sc 1-z z H x Transmittance 1 st st Hydrogen Hydrogen Loading Loading

17 Y 1-z Sc z H x Transmittance Subsequent Hydrogen Loading

18 Optical Switching in Y 1-z Sc z H x Fully hydrogenated Large decrease of transparency for z > 0.10 and dihydride-like behavior Unloaded, x ~ 2 Decrease in dihydride transparency peak

19 Optical Switching in Y 1-z Sc z H x Phase separation observed in peak splitting of XRD evident in trihydride-like transmittance of Y 0.60 Sc 0.40 H 2+δ QuickTime?and a BMP decompressor are needed to see this picture.

20 Optical Switching in Y 1-z Sc z H x Fully hydrogenated Estimation of the bandgap Extrapolate the transmission edge to zero: E g ~ ev T( ω)= T o exp C hω E g k B T ( ) ν T 0 contains absorption (of Pd and substrate) and interface reflections ν= 2 (allowed, indirect gap) E g (z=0.0) = 3.25±0.03 ev E g (z=0.1) = 3.32±0.12 ev

21 Optical Switching in Y 1-z Sc z H x Trihydride transmittance converges with dihydride Energy of maximum transmittance converges with increase in z

22 Electrical Properties of Y 1-z Sc z H x Increase in overall resistivity with Sc concentration 0 z 0.50 Decrease in overall resistivity with Sc concentration 0.50 < z 1.00 Magnitude of M-I transition is reduced as Sc concentration increases ()= ρ Y + ρ Sc ρ Y ρ z [ ( ) z]+4ρ do z1-z ( ) x 0, ρ do ~ µω cm x 2, ρ do ~ µω cm

23 Temperature Dependent Electrical Properties of Y 1-z Sc z H x 2 Y 0.50 Sc 0.50 H 2 ρ (µω 뷵 m) Y 0.90 Sc 0.10 H 2 Y 0.80 Sc 0.20 H 2 ScH 2 YH T (K) Unloaded dihydride films: metallic behavior for all alloys except z = ρ (µω 뷵 m) z = T (K)

24 Temperature Dependent Electrical Properties of Y 1-z Sc z H x z 0.10 : Insulating below orderdisorder transition 300 YH 3-δ ρ (µω cm) Y 0.90 Sc 0.10 H 3-δ YH 3-δ ρ (µω 뷵 m) Y 0.90 Sc 0.10 H 3-δ Y 0.5 Sc 0.5 H 2+δ Y 0.8 Sc 0.2 H 2+δ ScH 2+-δ T (K) Order-disorder disorder transition of hydrogen vacancies T (K) z 0.20 : metallic with low temperature impurity below order-disorder transition Shinar et al., Phys. Rev. Lett. 64, 563 (1990).

25 Temperature Dependent Electrical Properties of Y 1-z Sc z H x ρ (µω cm) 320 YH 3-δ 300 Y 0.90 Sc 0.10 H 3-δ 140 Y 0.80 Sc 0.20 H 3-δ ln(t) (K) YH 3-δ ρ vs ln(t) ( K) K Kondo scattering 2D layers w/ weak localization ln( ln(ρ) vs T -1/4 (4 20 K) K Mott insulator w/ VRH z 0.10 No reasonable fit ( K) K ln( ln(ρ) vs T -1/4 (4 20 K) K YH 3-δ 5.70 ln (ρ) Y 0.90 Sc 0.10 H 3-δ Y 0.80 Sc 0.20 H 3-δ T -1/4 (K -1/4 )

26 Temperature Dependent Electrical Properties of Y 1-z Sc z H x Shunt RE layer 310 Metallic ρ behavior ρ 4K = ρ 4K (YH 3-δ ) Recalculate for 10 nm film ρ ρ Pd ~ 29 µω cm ρ (µω cm) ln (ρ) ρ (µω cm) ln (T) T -1/4 (K -1/4 ) T (K)

27 Statistical Modeling and Discussion Fraction of octahedral sites with n Sc nearestneighbors 0.6 N n N = X! ( X n)!n! ( 1 z)x n z n 0.5 N n /N n = 0 n = 1 n = 2 n = 3 n = 4 n = 5 n = Scandium Concentration, z

28 Statistical Modeling and Discussion Octahedral site occupancy only when n = 0 x = 2 + (1-z)* N o /N Octahedral site occupancy only when n = 0 and 1 x = 2 + (1-z)* (N o +N 1 )/N If effective medium has volume averaged transmittance T α y in x = 2 + y Transmittance Calculated Maximum H Content Scandium Concentration, z n = 0 only n = 0 and n = n = 0 n = 0 and n = Scandium Concentration, z 1.0

29 Statistical Modeling and Discussion a) H b) YH 2 YH 3 H ScH 2 Phase diagram from calculations of hydrogen, x n = 0 n = 0 and 1 Dashed lines indicate regions of phase separation due to hydriding YH 3 YH 2 ScH 2

30 Summary and Future Directions Reduction of metallic-mirror to transparent-insulator transition z 0.20 V cell (z = 0.20) = V cell (Lu) = 177 Å 3 Dihydride/Trihydride forming limit Convergence optical transmittance properties with increased Sc concentration Large disorder (~120 µω cm cm) in metallic-phase resistivity MIT 0 < z < 0.10 temperature dependence z 0.20 temperature dependence as pure Sc Combinatorics result in understandable tri- to dihydride behavior Future Exploring the region 0 < z < 0.2 Epitaxial,, substrate heating, and studies to identify phase separation and strain effects XRD of x = 3 Electrochemical loading apparatus STM and/or AFM for in-situ loading topography and spectroscopy Y-Lu-H system

31 Acknowledgements Advisor & Coworkers John T. Markert Casey W. Miller Yong Lee Jae-Hyuk Choi Utkir Mirsaidov Funding U.S. National Science Foundation Robert A. Welch Foundation Texas Advanced Technology Program Discovery Vrije University, Amsterdam, Holland Family & Friends

32 XRD of Y 1-z Sc z Alloys θ θ d 2θ nλ = 2d sin θ Bragg condition gives peaks when the path difference is an integer multiple of the wavelength of incoming x-rays. Thin films give less reflected intensity and have small crystallites

33 Y 1-z Sc z H x Transmittance 1 st Hydrogen Loading Y 1-z Sc z H x Transmittance 1 st Hydrogen Loading Y Y Sc 0.10 Y 0.80 Sc 0.20 Y 0.50 Sc 0.50 Y 0.20 Sc 0.80 Sc

34 Y 1-z Sc z H x Transmittance Subsequent Hydrogen Loading Y Y 0.90 Sc 0.10 Y 0.80 Sc 0.20 Y 0.50 Sc 0.50 Y 0.20 Sc 0.80 Sc

35 Y 1-z Sc z H x Transmittance 1 st Hydrogen Loading Y 0.25 Sc 0.75 Y 0.10 Sc 0.90

36 Y 1-z Sc z H x Transmittance Subsequent Hydrogen Loading Y 0.25 Sc 0.75 Y 0.10 Sc 0.90