Radiation Defects in Alkali Halides and Oxides

A.I. PopovInstitute of Solid State Physics, University of

Latvia, LV

REI-15, Padova, Sept.1, 2009Basic Properties of Radiation-

Induced Point Defects in Halides and Oxides

A.I. Popov, Max Planck Institute, Stuttgart and Institute of Solid State Physics, University of Latvia, LV

E.A. Kotomin, Max Planck Institute, Stuttgart and Institute of Solid State Physics, University of Latvia

J. Maier, Max Planck Institute, Stuttgart

The is no doubt that F center in AHC may be decribed as an electron trapped on anion vacancy.

Optical absorption by F centers in alkali halides

1.Shape of the band is single Gaussian in almost all AHCK=K0exp[-a(hmax -h)2

2. The half-width depends on T[H(T)/H(0)]2=coth[h/2kT)3. It was found experimentally that in alkali halides for F-band absorptionthe relation Eabs= 16.75 eV/(a Å)1.772

holds quite well!

Radiation Defects

Ionizing radiation produces a variety of vacancy and intersitial type of point defects:

In alkali halides: vacancy defects includes bare cation and anion vacancy, as well as halogen vacancy with one electron ( or F center).

KCl - The activation energy for diffusion is found to increase monotonicallyin the series Vc, Va and F center 1.19 eV, 1.44 eV and 1.64 eV

In simple oxides: vacancy defects includes bare cation and oxygen vacancy, as well as oxygen vacancy with one or two electrons (F+ and F center).

MgO- The activation energy for diffusion is found to increase monotonicallyin the series Vc, Va, F+ and F center (2.43, 2.50, 2.72, and 3.13 eV, respectively).

Type of Defect Alkali Halides Oxide Other II-VI

Anion Vacancy V+ = simple vacancyV0 = FV- = F

V++ = simple vacancyV+ = F+

V0 = F

F, F+ and simplevacancy reported(MgS, CaS, SrS,BaS

CationVacancy

V- = simple vacancyV0 = VF

V2- = simple vacancyV-

V0

V2- = simplevacancyV-

V0

Polyvacancies DivacancyM = F2

M+= F2+

M = F2

-

R = F3

R+ = F3+

R = F3

-

N = F4 etccolloids

Anion vacancyaggregatesF2

F2+

F22+

nanocavities

Intersitials I0 = H centerI+ = simple cation intersitialI- = simple anion intersitial

O2- in fluoritestructure oxides

Other intrinsicdefects

Self-trapped holeSelf-trapped excitonV2 and V3 centers

Small polarons insome systems

Radiation Damage Processes

1. Electronic processes 2. Elastic collisions Five types of radiation may produce displaced atom or ions (1) - rays, (2) energetic electrons, (3) thermal neutrons, (4) fast neutrons, (5) energetic atoms or ions

3. Radiolysis(1) Electronic excitation creation of an electronic defects(2) Conversion of this energy into kinetic energy of a lattice ion ion moves(3) The motion and stabilization of the ion

The available energy, Egap (in fact Ex < Egap) > the formation energy of the Frenkel pair.

the radiolysis can only occurs in insulators or wide band-gap semiconductors .The excitation must be localised on one atomic (or molecular) siteNon-radiative transitions, allowing an efficient kinetic energy transfer to an atom,

must prevail over radiative transitions

Could work inalkali halides(anions and

cations)alkaline-earth

halides

Difficult inoxides

Elastic collisions

Defect Production rate as a function of irradiation energy for MgO under electron irradiation.The damage rate is strongly dependent on the energy.Threshold for radiation damage.

For relativistic particles such as electrons, the maximumenergy Td (in eV) transferable from an incident electron of energy E (in MeV) to a lattice ion of mass number A is given by:

Td =2147.7E(E + 1.022)/A

Displacement energy

Other materials:II-VIZnS 7-9/15-20ZnSe 7-10/6-8CdTe 6-9/5-8CdSe 6-8/8-12III-VGaAs 9/9.4InP 6.7/8.7InAs 6.7/8.3Group IVC 25 graphite 35-80 diamondSi 13Ge 13-16

F-H pair Formation in alkali halides: Self-trapped Exciton F-H pair

Resistant: Metals, semi-conductors. crystalline Oxides: metastables (SrTiO3, MgO, Al2O3, c-

SiO2) Sensitive: Alkali halides Alkaline-earth halides CaF2, MgF2, SrF2 : KMgF3, BaFBr, LiYF4: Silver halides AgCl; AgBr Amorphous solids a-SiO2 , a-As2Se3, a-As2S3, a-Se, a-

As

Water and organic mater (bio matter)

Resistant and sensitive materials

Radiolysis versus ballistic damage Radiolysis is not universal, not easily predictable

2) Is in essence temperature dependent

3) Spans over a wide time scale

4) Occurs generally on one sub-lattice (anions)

5) Radiolysis occurs occasionally when it occurs, it is with a good energetic efficiency.

Elastic damage occurs every time but with a relatively poor energetic efficiency.

Charge-carriers self-trapping

STE:BeO-YAGMgO, Al2O3

Self trapping of charge carriers results from a competition between deformation and polarisation of the lattice

Radiation Defects

1.Electronic defects, which involve changes in valence statesExamples: KCl:Tl+

Tl+ + hole Tl2+

Tl+ + electron Tl0

MgO:Fe etcFe2+ + hole Fe3+

Fe3+ + electron Fe2+

n-irradiated MgO

In this talk:

1. F center production in Cs-halides. Show the extension of Rabin-Klick diagram for all AHC.

2. Discuss differences between F center in AHC and F+ and F center in oxide materials (MgO as an example)

3. Discuss whether common and famous Mollwo-Ivey rule could be extended for oxide materials

Type Self-trapping Formation of defects Some

exciton hole Singleexcitation

Denseexcitation

examples

1 no no no yes MgO, CaO

2 yes yes yes yes Alkali halides

RABIN AND KLICK DIAGRAM

P D Townsend 1973 J. Phys. C: Solid State Phys. 6 961-966

Data for Cs-halides with CsCl-structute are absent !!!

CsI

Three different types of CsI crystals were studied in this paper.

Nominally pure CsI crystals have been grown in the Laboratoire de Spectroscopie Atomique (CNRS/ISMRA, Caen).

The low-doped CsI–Tl crystals with Tl+ ion concentration of about 1017 ion/cm3 have been supplied by Dr. P. Schotanus (SCIONIX, Holland).

The highly doped CsI–Tl with Tl+ ion concentration of about 1019 ion/cm3 was obtained Institute of Solid State Physics, University of Latvia.

Crystals have been irradiated at GANIL on the medium-energy beam line (SME) with 86Kr ions (8.63 MeV/amu).

In this study, both the irradiation and in-situ measurements were done at 15 K.

F centre production in CsI crystals under ion irradiation at 15 K

86Kr ions (8.63 MeV/amu)

Production efficiency (eV/centre) of F band absorption for all cesium halides. CsCl - 7 × 103 eV/centre S/D=0.43CsBr - 8 × 102 eV/centre S/D=0.32CsI - 2.5 × 107 eV/centre S/D=0.17

Evolution of the optical absorption spectra of CsI under irradiation at 15 K with fluences1011 ions/cm2 (1); 3 × 1011 ions/cm2 (2); 6 × 1011 ions/cm2 (3); 9 × 1011 ions/cm2 (4); 1.2 × 1012 ions/cm2 (5); 1.6 × 1012 ions/cm2 (6); 2.0 × 1012 ions/cm2 (7).

0,0 5,0x1011 1,0x1012 1,5x1012 2,0x1012

0

20

40

60

80

100

15 K

CsI-Tl (highly-doped)

CsI (pure)

CsI-Tl (low-doped)

ab

so

rpti

on

(a

rb.u

nit

s)

fluence

EXTENSION of The Rabin and Klick diagram

Photoconversion of F+ centers in neutron-irradiated MgO

Experiments and theory demonstrate that photon excitation of the positively charged anion vacancies at 5.0 eV releases holes that are subsequently trapped at V-type centers, which are cation vacancies charge-compensated by impurities, such as Al3+, F−, and OH− ions. A photoconversion mechanism occurs very likely via electron transfer to F+ centers from the quasi-local states which are induced in the valence band. INDO quantum chemical simulations of F+ centers confirmed the appearance of two induced quasi-local states located at 1.2 and 2.0 eV below the top of the valence band.

Hole Centeres in MgO

V- center -hole trapped on an oxygen neighboring a cation vacancy.

They are produced byUV-light, X-rays, or low-energy ions

Optical absorption band at 2.3 eV

A half-life time at RT:2-7 year

Hole Centeres in MgO

V0 center -two hole trapped on an oxygens neighboring a cation vacancy.

Optical absorption band at 2.36 eV

A half-life time at RT:10 hours

Hole Centeres in MgO

Impurity-related V center holes are trapped oxygens neighboring a cation vacancy, which are charge compensators for impurities (OH-, F-, Al3+, Si4+ etc)

Hole Centeres in MgO

Centre OA(eV) Half-lifeV- 2.33 yearsV0 2.36 10 hVAl 2.33 10 -15 hVF hoursVOD

VOH 2.21 10 hNa0 1.51

Photoconversion of F+ centers in neutron-irradiated MgO

3296 cm-

3323 cm-

Differential spectrum of the n-irradiated MgO crystals before and after UV irradiation for 50 min.

Fe2+ +h+ → Fe3+.

During thermal annealingconversionF center colloid band

NaCl, KCl, KBr etc

350 T 500 K

MgO, Al203 etc ?????

MgO TCR samples

The MgO crystals used were grown at the Oak Ridge National Laboratory using the arc fusion

technique.

The starting material was MgO powder from the Kanto Chemical Company, Japan.

TCR was performed in a tantalum chamber at 2000 K and 7 atmospheres of magnesium vapor, followed by rapid cooling. This process produces anion oxygen vacancies, due to a stoichiometric excess of cations.

MgO: vacancy diffusion

MgO- The activation energy for diffusion is found to increase monotonicallyin the series Vc, Va, F+ and F center (2.43, 2.50, 2.72, and 3.13 eV, respectively).

Dynamics of F-center annihilation in TCR MgO

F concentration(a) sample N-1 2 x1017 cm-3 Activation energy = 1.9 eV(a) sample N-2 2 x1017 cm-3 Activation energy = 2.5 eV(c) sample N-3 5 x1018 cm-3 Activation energy = 3.4 eV

To explain these observations, we suggest that a direct manifestation of the intrinsic diffusion of F centers is their diffusion-controlled aggregation to ultimately form

nano cavities in the temperature range of 1400±1650 K.

Eact is 3.4 eV which agrees well with the theoretical energy (3.1 eV) of the F-center elementary jump

Eact values of 1.9 and 2.5 eV are significantly lower and hence can not be attributedto migration of single F-centers. Thus, in samples MgO I and MgO II oxygen vacancies are annihilated either by forming dimer centers with selected impurities, which favours their joint diffusion to internal sinks (such as dislocationsand grain boundaries) or with more mobile defects (such as magnesium vacancies)

Mg vacancy + F-center ionised F center

Dynamics of F-center annihilation in TCR MgO

F concentration(a) sample N-2 2 x1017 cm-3

(b) (c) sample N-3 5 x1018 cm-3

Normalised concentration of (a) F centers in sample MgO II,(b) F centers in sample MgO III, (c) 3.59±3.35 eV absorptionband in MgO III against isochronal annealing temperature.

Assuming a first order kinetics, an activation energy for F-center diffusion was estimated forsample III to be 3.4 0.6 eV, in good agreement with theoreticalcalculations

Dynamics of F-center annihilation in TCR MgO

5 x1018 cm-3

Unexpected Results:brown coloration due to abroad extinction band centered at 3.59 eV (345 nm).

As the annealing temperature increased, the band became more intense, as it shifted toward lower energy. The band ultimately peaked at 3.35 eV It reached maximum intensity at 1673 K.

Nanocavities formation in MgO

exp=345 nm

From Mie theory:exp=320 nm

This extinction band has been attributed to Mie scattering from nano-size cavities with typical dimensions of 3 nm, coated with magnesium

metal.

Electron microscopy: TCR sample after annealing at 1673K in a reducing atmosphere.

Areas with a high concentration of dislocations were separated by regions in which only small rectangular features are observed

Specimens for TEM studies were prepared by mechanical grinding, dimpling, and argon ion-milling with an acceleration voltage of 5 kV and an incident angle of 10°.TEM, x-ray microanalysis, and electron diffraction studies were carried out in a Philips CM200 field-emission analytical electron microscope operated at 200 kV and equipped with a Be specimen holder.

Optical absorption by F centers in alkali halides with NaCl structure

F center in AHC was decribed as an electron trapped on anion vacancyIt was found experimentally that in AHC for F-band absorptionthe relation

Eabs= 16.75 eV/(a Å)1.772

holds quite well!

Particle–in-a-box type model:E=3.14(i2+j2+k2)/2a2

Transition energy from GS(i=j=k=1)to the first excited state(2,1,1); (1,2,1) or (1,1,2) is given as

Ea = 3(3.14)2/ 2a2

Particle–in-a-box type model --->Electron in halogen vacancy

1,5 1,6 1,7 1,8 1,9

0,2

0,4

0,6

0,8

1,0

15 K

1 - CsI-Tl (low-doped)

2 - CsI - pure

21

op

tic

al

de

ns

ity

energy (eV)

Optical absorption by F centers in alkali halides

with CsCl structure

4 4,5 5

CsI

CsBr

CsCl

2.2

2.0

1.8

1.6

En

erg

y (

eV

)

Lattice constant (Å)

Comparison of LiF and MgO

Mollwo-Ivey rule(extension)

It was found experimentally that in alkali halides for F-band absorptionthe relation Eabs= 16.75 eV/(a Å)1.772

holds quite well!

It works also foroxides (MgO, SrO, CaO)sulfids (CaS, SrS, BaS)

This confirm: Particle–in-a-box type model --->Electron in halogen (or oxygen, or sulphur) vacancy

Optical absorption spectra of MgO crystal

1) after TCR

2) after subsequent uv irradiation

3) after neutron-irradiationMgO crystal up to a dose of 6.9·1018

neutrons/cm2

Conclusion:

1. F center production in Cs-halides. Show the extension of Rabin-Klick diagram for all AHC.

2. Discuss differences between F center in AHC and F+ and F center in oxide materials (MgO as an example)

3. Show that famous Mollwo-Ivey rule could be extended for some simple oxide and sulfide materials with NaCl structure

Thank you very much for your attention