Growth and properties of monocrystals for miniature lasers

Prog. Crystal Growth Charact. Vol. 3, p p . 2 b - 7 - 2 7 1 . Pergamon PressLtd. 1981. Printed in Great Britain

0146-3535/81/0101-02b"/~O5.00/0

GROWTH A N D PROPERTIES OF M O N O C R Y S T A L S FOR M I N I A T U R E LASERS B. N. Litvin, K. Byrappa and L. G. Bebich Department of Crystallographyand Crystal Chemistry, Faculty of Geology, Moscow State University, Leninsky Gori, Moscow B-234, U.S.S.R.

(Submitted 8 September 1980)

ABSTRACT We have considered the special features of growth for the Nd compounds in the form of monocrystals containing a high concentration of Nd ions (>I021 ions/cm 3) with an anamolously low concentration quenching. These compounds are known popularly as "nezoites" containing isolated Nd polyhedra in the structure, which gives a clue to the technology of growth. All the known nezoite crystals, their X-ray analyses and the conditions of growth from multicomponent systems are tested. At present there are two well-known methods of growing nezoites: (I) by evaporation of the solvent at constant temperature and pressure; and (2) by flux method. The authors have discussed the advantages and disadvantages of these two methods. The problem of the presence of light extinguishing admixtures, particularly (OH) molecules in the crystals, and the influence of these admixtures on the basic properties of these crystals (nezoites) is discussed in detail.

I.

INTRODUCTION

With the recent developments in fibre-optic communication, there is a problem with the miniature sources of light, particularly lasers based on the dielectrics. Until recently, these materials contained a low concentration of active ions, which was a major obstacle in the attempts to use them in optoelectronics. In 1973 laser beams were obtained from a NdP5014 crystal (1) containing 3.86 x I021 ions of Nd/cm 3. Such a high concentration of active ions lead to the further decrease in size of the laser crystals to tens of micromillimetres (2,3). An analogous effect was found in LiNd(P03) ~ monocrystals (4-6). Thus, the possibilities of developing miniature lasers in the crystals containing a high concentration of Nd ions was proved. At present, there are more than 20 suitable Nd compounds (Table I), which are different from other Nd compounds with an anamolously low concentration quenching. The reason for such a low concentration quenching is the result of the isolated Nd polyhedra in the crystal structure leading to a weak interaction of the Nd ions (25,26). The compounds, wherein the rare-earth ions go in the form of islands and their polyhedra are nut interconnected with each other in the structure,

257

NdP5014

NdTa7019

(NH~)Nd(P~O12)

KNdP~012

RbNdP~O12

CsNdP~012

LiNd(P03) ~

NaNd(PO3) ~

KNd(P03) ~

RBnD(P03) 4

a-CsNd(P03) 4

B-CsNd(PO3) 4

TINd(P03) 4

Na3Nd(P04) 2

Na3Md(V04) 2

NasNd(W04) 4

Na2Pb6Nd2(P04)6C12

K3Nd(PO~) 2

K5Nd(Mo04) 4

AlzNd(B03) ~

Li2KsNdF10

2

3

4

5

6

7

8

9

IO

II

12

13

14

15

16

17

18

19

20

21

Nezoite

I

No.

Rhomb.

Trigl.

Monocl.

Monocl.

Hex.

Tetra.

Monocl.

Monoel.

Monocl.

Monocl.

Monocl.

Monocl.

Monocl.

Monocl.

Monocl.

Cub.

Honocl.

Monocl.

Monoel.

Tetra.

Monocl.

System

Pror~

R32

10.440

P21/n Pbc21 PL~c21 I41/a Pb3/m P21/m P2/m 20.65

9.3416

10.358

9.532

9.946

11.559

15.56

]5.874

7.145

10.463

10.461

7.280

9.907

9.844

P21

I2/c P21/n P21/ P21/n P2I/b

15.100

7.845

7.828

7.916

9.79

8.77

P21/c C2/c C2/c C2/c

a

Space group

7.78

-

17.940

5.631

-

-

14.2

13.952

8.050

8.809

13.008

9.04l

8.436

13.10

7.008

-

]2.691

12.680

12.647

-

8.99

6.90

7.306

14.30

7.444

7.287

11.453

19.48

18.470

11.007

9.176

9.028

10.983

8.007

7.201

13.25

-

10.688

10.650

10.672

12.944

13.03

Cell parameters b c

Table I. X-Ray Table for Nezoites

90.00

B=103.98

Bffi 9 0 . 9 5

90.00

90.00

B=105.86

Yffi 9 9 . 6 5

Tffii24.07

Bffi106.16

Bffi 9 1 . 9 7

B= 9 0 . 5 1

Bffi 9 0 . 1

Bffi112.34

Bffill2.00

B=110.34

-

90.48

axial angle

1108.5

2577.23

399.49

528.1

1530.46

4304.14

4090.62

889.83

569.36

1017.6

997.26

491.45

934.52

913.16

3442.95

984.23

976.64

1001.79

1240.61

I027.3

V (A 3)

4

3

6

2

1

4

24

24

4

2

4

4

2

4

4

12

4

4

4

4

4

Z

3.78

3.75

4.10

3.92

3.45

3.89

3.63

3.37

3.43

3.39

3.43

3.68

3.37

3.17

3.38

d (gm/cm3)

24

23

22

21

20

19

18

17

16

12

12

16

15

14

13

12

I1

10

9

8

7

Ref.

o

w

=

Monocrystals for Miniature Lasers

26O

are called "nezoites". For t h e compounds having an anamolously low concentration quenching, the term nezoites more correctly reflects their crystal chemistry than the term "stoichiomatric compounds" as proposed in Ref. 27. In the structure of any crystal from Table I we can easily distinguish an nezoite complex made-up of isolated Nd polyhedra, and ligands often having tetrahedral ions (Fig. I). The characteristic feature of such a complex is the bonding of Nd polyhedra by ligands along its long axis, so that the Nd polyhedra in the structure of nezoites always exist in a state of inter-isolation. This property of nezoites throws a light on the technology of obtaining them in the form of monocrystals.

Fig. I.

Nezoite complex made up of dodecahedral centres (Nd ions) and tetrahedral ligands ([TO~], where TfS,P,W,Mo and others).

2.

GROWTH OF NEZOITES

The growth of optically perfect monocrystals for miniature lasers does not carry any sort of specificity and it varies little from the general problem of growing high quality crystals for other purposes. The differences lie in the technology of growth, which should maintain: (]) the existence of Nd ions in the isolated form (inter-isolated state); and (2) the lowest possible amount of hydroxyl and other impurities which quench the optical emission in the crystals. I n t h e l i q u i d , from which t h e n e z o i t e s grow, Nd s h o u l d be d i s s o c i a t e d t o form individual ions. F o r t h i s p u r p o s e we u s u a l l y u se c o n c e n t r a t e d s o l u t i o n s or alkaline melts. However, t h e p r o c e s s o f Nd d i s s o c i a t i o n i n t h e s e o f t e n c a r r i e s a r e v e r s i b l e c h a r a c t e r and t h e p h a s e s d e v e l o p w i t h Nd i o n s i n t h e condensed p a t t e r n d u r i n g t h e c r y s t a l l i z a t i o n -- f o r example, as i n NdP309 (28) and NaNdSiO~ ( 2 9 ) . In order to avoid t h i s i t is necessary t h a t in the process of c r y s t a l l i z a t i o n t h e Nd i o n s a r e to be h e l d i n a d i s s o c i a t e d s t a t e -- i . e . i n t h e form of a n e z o i t e complex. A s h e l l o f l i g a n d s around Nd p r e s e r v e s t h e i o n s from i n t e r - c o n d e n s a t i o n and a l l o w s them t o e n t e r t h e c r y s t a l l a t t i c e as i s o l a t e d i o n s . These l i g a n d s o f t e n o c c u r as t e t r a h e d r a l o x y - a n i o n s [TO~] i n th e f o r m a t i o n o f a n e z o i t e complex o f t y p e NdT802~ , where T - S, P, W, Mo, e t c . Thus, t o o b t a i n n e z o i t e c r y s t a l s , i t i s f i r s t n e c e s s a r y t o b r i n g Nd i o s n i n t o a d i s s o c i a t e d s t a t e , f o l l o w e d by c r e a t i n g t h e c o n d i t i o n s f o r t h e f o r m a t i o n o f n e z o i t e c o m p l e x e s , and o n l y t h e n does t h e d e s i r e d c r y s t a l l i z a t i o n take place. The most s u i t a b l e s y s t e m s f o r t h i s p u r p o s e a p p e a r s t o be Nd c o n t a i n i n g ternary systems of type Me20-Nd203-TxOy , wherein Me ffiH, Li, Na, K, Rb, Cs and TI or their admixtures. Table 2 shows examples of such admixtures and the conditions for the growth nezoites.

There are two examples showing the growth of nezoites from binary systems: NdP501~ in the system Nd203-P205 (30), and NdTaTOl9 in the system Nd203-Ta205 (8). In both cases, the growth of even small crystals take place under highly specific conditions of increased pressure. The process of crystallization is much more simple in the multicomponent systems. The quantitative ratio of the components largely

B. N. Litvin, K. Byrappa and L. G. Bebich

2~

determines the stoichiometric nezoitic complexes in these systems; i.e. the ratio of TzOy/Nd203 should not be less than 8. The main problem of concern is the quantity of some components, which are incorporated in more than stoichiometric amounts. As a result, the molar ratio of the nutrient components in the growth of nezoites is not regular and can be expressed approximately as Me20(Me)/Nd203/ TzOy = 2 > | < 8. The additional amount of the components are introduced into the nutrient depending upon the individual cases. Thus, the growth of nezoites requires a definite surplus of TxO H components, and they may not be taken from the stoichiometric melt. Thegefore, in the growth of nezoites we use high temperature aqueous solutions or flux. Similarly, in the synthesis of many phosphates in the form of crystals, we use highly concentrated solutions of phosphoric acid. It is considered that at over 250 C, orthophosphoric acid converts to pyrophosphoric, and with a further increase in temperature it transfers to an even more condensed state due to the expulsion of water (42). Figure 2 represents a temperaturecomposition diagram for the system H20-P205 at pressures up to |.0 atm (43). In

8OO

600

4.00

2

200

20 Fig. 2.

40

60

80

IO0

Temperature-composition diagram for the system H20-P205 .43 (i) vapour (P205 + H20) ; (ii) liquid (H3PO4 + H20 ).

region (2) of the diagram, crystallization can be carried out at normal pressure, whereas the crystal growth from solutions corresponding to the composition of the region (1) requires the use of an hydrothermal apparatus. Oxides of rare earth such as Nd203 shows a negative temperature coefficient of solubility in the phosphoric acid (44). This acts as a clue to determine the technological features of growing monocrystals of rare earth phosphates in general. The starting solution can be prepared in the following way: a required amount of Nd203 has to be dissolved in the 85% phosphoric acid at a temperature not less than 250°C, until the complete homogenization of the solution take place. Later, it has to be brought into the supersaturation by increasing temperature or as a result of evaporation. In our experiments for the growth of Nd nezoites we use an apparatus shown in Fig. 3. The solution was held in a vitreous carbon crucible kept inside a vitreous carbon cylinder. The temperature was initially raised to 250-300°C and held for 1/2 or I day, until the necessary equilibrium develops between the vapour and solution. Later, the temperature was raised to a predetermined point. During the crystallization, if there is a need for P205 vapour, a valve can be opened at the top. The process of evaporation of the solvent in the crucible (4) can be controlled by varying the magnitude of the opening in the cap, the temperature of growth, and the rate of flow of P205 vapour from the space over the crucible. The starting solution in the growth of Nd pentaphosphate, for example, was prepared by dissolving Nd203 in an 85% H3PO 4 in the ratio of I:10-I:|5; afterwards the surplus water from the solution was removed by evaporation at 250-320°C for

Monocrystals for Miniature Lasers P2Os+ H20

c

I[

Fig. 3.

t I Ii I Ii k Ill I II r ill Ii Ii t II I ]l t I I

I1~111 ' ~ I I t

Crystallization chamber for the growth of Nd phosphate monocrystals by evaporation of the solvent. (I) chamotte bricks (fire clay bricks); (2) temperature-isolation material (asbestos, mullite cotton); (3) ceramic stand (eluminia) ; (4) vitreous carbon crucible; (5) vitreous carbon glass with cap (diameter of the opening determines the rate of evaporation); (6) stainless-steel vessel; (7) heater; (8) opening for the thermocouples; (9) germitization gasket; (]0) stainless-steel cap; (]]) vapour (P205 + H20) outlet; (]2) water vapour inlet (stainless steel); (]3) evaporator; (14) compzessor for air (air compressor); (15) heater (lS-100°C). T 1 and T 2 chromel-alumel thermocouples.

]2-24 hr. This homogeneous solution was later preheated to a predetermined temperature by slowly raising from 250 to 320°C, and the constant evaporation of the solute produces crystals. We carried out the growth of Nd pentaphosphates in rhe four-component systems (Table 2). This led to a decrease in the rate of evaporation of solvent at temperatures higher than 550°C, to an increase in the solubility of pentaphosphate, and to changes in the crystal habit in the required direction. In the systesm Me20-Nd203-P205, wherein Me = Na, K, Rb and Cs, the appearance of mixed polyphosphates, whereas in the systems MeO-Nd203-P205-H20 , where Me = Ca, Sr, Ba and Pb, the formation of the pentaphosphate is limited by an appearance of highly viscous solutions (Fig. 4). We carried out the growth in the four-component systems within the stable region of pentaphosphate mainly by evaporation of the solvent, since the solubility of pentaphosphate in such systems depends less upon temperature than in the three-component systems. In the growth of mixed phosphates MNd phosphates, where M = Li, Na, K, Rb and Cs) the alkaline ions are added to the solutions in the form of, respectively, carbonates and Nd in the form of Nd203. These carbonates are selected according to the ratios shown in Table 2, and they dissolve in the phosphoric acid at room temperature. Since the mixed Nd phosphates show a negative temperature coefficient of solubility, the growth can be carried out by slow heating of the solution with a controlled partial pressure of water. We have obtained these crystals by another method called "counter-flow diffusion" of the starting components. This method is nearer to the method of obtaining crystals from gels (45). The supersaturation in the counter-flow diffusion occurs as a result of the chemical reaction: X Me20 + ~ Nd203 + Z P205 -~ M2xNd 0X+3/,+5Z

C F F F F F F F F

E,S E~S S

Nd203-P205-H20

Li20-Nd203-P205

Na20-Nd203-P205

Na20-Nd203-P205

Na20-Nd203-V205

K20-Nd203-P205

Na20-Nd203-NO3

K20-Nd203-P205

K20-Nd203-MoO 3

LiF-KF-NdF 3

Me20-Nd203-P205-H20

(~=Na,K,Rb,Cs) ~O-Nd203-P205-H20 (Me-Ca,Sr,Ba,Pb)

Sb203-Nd203-P205-H20

NdP501~

LiNd(P03)~

NaNd(P03)q

Na Nd(PO )

Na3Nd(VOk)2

KNd(POq) 3

Na5Nd(W0~)~

K~Nd(PO~)2

K5Nd(MoO~)k

Li2K5NdF10

NdPsO1w

NdP501~

NdP501W

C

Nd203-Ta205

NdTa7019

F

C

Method

Nezoites of Complex

(3-I):I:(20-40):(30-50)

(I-6):I:(20-40):(30-50)

(8-12):I:(10-40):(15-60)

(12-14):I:I0

(8-10):I:(4-6)

3:1:12

(6-8):I:I0

(6-8):I:I0

5:1:9

(2-3):I:6

I:(30-60):(40-80)

1:7

I < I0

Molar ratio of components

for the Growth of

Nd203-P205

System

Conditions

NdP501~

Nezoite

Table 2. for growth

T - 350-620°C

T = 550-680°C

T - 320-680°C

T = 800 ÷ 400°C, V - 9°C/hr.

T = 980 ÷ 620°C V = l-2°C/hr.

T = 1000 + 650°C V = 5°C/hr.

T = If00 ÷ 700°C V = 5°C/hr.

T = 900 ÷ 700°C F ~ 2°C/hr.

T - ll00 ÷ 850°C V - 2-5°C/hr.

T - If00 ÷ 850°C V - 2-5°C/hr.

T = 1000 ÷ 700°C V = 2°C/hr.

T = 950 ÷ 650°C V = 0.l-l°C/hr.

T = 320-580°C

T - 1200-1500°C, p = 600 arm.

900°C

Conditions

Systems

u

a

a, 38

24

22, a

21

19, a

15

a

]7, a

14, 37

35, 36

31-34

8

30

Ref.

o" pJ. o=c

c~

c~ t~

m

m .o

t~

t=+ rt

c~

t~

K20-Nd203-P205-H20

Rb20-Nd203-P205-H20

Rb20-Nd203-P205-H20

Cs20-Nd203-P205-H20

Cs20-Nd203-P205-H20

Cs20-Nd203-P205-H20

BaO-AIzO3-Nd203-B203

R-AI203-Nd203-B203 (R=PbF2,MoO3,KSO b and other admixtures)

NaCl-PbO-Nd203-Pz05

RbNdPqOl2

RbNd(P03) ~

CsNdP~OI2

a-CsNd(P03) ~

~-CsNd(P03) ~

AI3Nd(B03) ~

AI3Nd(B03) ~

Na2Nd2Pb2(P04)6CI2

V = rate of cooling.

T = temperature.

E - crystallization from solution by evaporation.

C = by chemical reactions.

D = counter-flow diffusion method.

F

F

F

S, DjC SjDjC SjDjC

(4-5):1:(12-16):(15-25)

SjD~C SjD, C SjDjC SjDaC

T = 1200-850°C V = 0.5-1.5°C/hr. 800-400°C V = 9°C/hr.

(2-6) 3 : ! : 4

(8-6) 2 : ! : ( 2 - 3 )

=

T = 1200-900°C V = 0.5-1°C/hr.

T

T = 670°C

(3-8):1:(9-35):(12-45)

T = 520-670°C

T = 350-520°C

T = 350-750°C

T = 350-600°C

T ffi 300-620°C

T ffi 280-320°C

Conditions for growth

2:2:1:9

(I-i0):1:(8-40):(10-50)

(2-10):1:(7-100):(]0-140)

(|-15):|:(5-35):(8-45)

(2-14):1:(7-35):(10-45)

2:1:(15-25):(20-30)

Molar ratio of components

Method

S = crystallization from solution by slow heating.

F ffi flux method.

K20-Nd203-P205-H20

KNdPqO12

System

KNd(PO3)q

Nezoite

Table 2 (Continued)

20

23, 40 41

39

a

u

a

a

u

a, 38

u

Ref.

r~

~° m rt

o

rt

o o o

B. N. Litvln, K. Byrappa and L. G. Bebich MezO

MeO

.o.o#°'°.,' (a)

Fig. 4.

xo-~o~ (b)

Stable region of Nd pentaphosphate in the fourcomponent system (w = region of super viscous solutions).

Addition of the components to the growing crystals takes place continuously without any fluctuations, since the process on the whole does not require any changes in the external parameters. Only small changes in temperature occur in the crystals growth and the rates of reaction and diffusion depend less upon temperature. As a result of this, we can grow optically high quality crystals successfully. The basic problem in the growth of laser crystals from aqueous solutions is connected with the presence of hydroxyl ions in the growing crystals. Intrusion of hydroxyl ions into a crystal framework takes place due to the development of weak centres. As a result of this, crystals are unsuitable for the use in laser techniques. In this case, it is necessary to take into account the existence of (OH) in the order of -1019 hydroxyl group/cm3; i.e. of a magnitude, which cannot be ascertained by the usual methods of analysis. Until recently, crystals with such an amount of (OH) were considered to be waterless, and they were successfully grown from hydrous solutions. But the requirements for laser techniques imposed more rigid restrictions for these materials with reference to the (OH) admixture in them. The growth of Nd pentaphosphate monocrystals remains as an unsolved problem, whether or not it is possible to grow laser materials having fluorescent lifetimes (T) = 120 ~sec from aqueous solutions. However, such results occur in crystals grown at temperatures higher than 550°C. In crystals grown at much lower temperatures, the fluorescent lifetime falls to =30~i~ sec (46,47). We checked these results in pentaphosphate crystals obtained at temperatures of 320-680°C. The presence of (OH) ions in the crystals was monitored by measurements of absorption bands in i.r.spectra in the region 3000 cm -l, and fluorescent lifetime (T) (Table 3). It can be seen that the value of T changed from 30 ~sec at 320°C, to 120 sec at 680°C. Similarly, the coefficient of absorption (=) in the region 3000 cm -l changes from 45 cm-I in crystals obtained at 320°C, to 2-4 cm -l in crystals grown at 680°C. This is direct evidence of the changes in (OH) content in the crystals. Apart from the Nd pentaphosphates, even the mixed Nd phosphates show a similar property. For example, KNd tetrametaphosphate crystals obtained at 320-350°C have T = 115-125 ~sec, whereas KNd tetrametaphosphate crystals obtained at 300°C have T z 65 ~sec, and the crystals grown at 500°C have ~ = II0 ~sec. Similar results are obtained for Cs and Rb tetrametaphosphates (Table 3). From these results it can be concluded that the (OH) concentration in the crystals determines the molar fraction of water in the system; it also determines special features of existence of the (OH) ions in the crystal lattice. Efforts to reduce the molar fraction of H20 in the growth of Nd pentaphosphate and KNd tetrametaphosphate did not give any positive results; the changes in the value of ~ were observed, but were very insignificant. Hence, the presence of (OH) ions mainly depends upon the crystal lattice. Due to the lack of corresponding investigations about the presence of (OH) ions in natural crystals, prior decisions based on the temperature of growth are impossible. The question of the possibility of obtaining laser crystals from aqueous solutions at low temperatures can be answered positively. However, the concentration of (OH)

265

Monocrystals for Miniature Lasers

in crystals depends upon temperature, molar fraction of water in the system, and its partial pressure. To obtain high quality monocrystals for miniature lasers it is necessary to carry out the growth under constant temperature and pressure, and for this there are two methods: (1) growth by counter-flow diffusion of starting components; and (2) growth by evaporation of the solvent. The authors have grown crystals of Nd pentaphosphate by the first method (30). Into a fluoroplast-4 (teflon) vessel the starting solutions (V~z. pyrophsophoric acid, d = 2.2 gms/cm 3 and orthophosphoric acid with Nd in it) were poured carefully, so that a boundary develops between these two solutions. The vessel was held at 250-270°C for a period of 10-12 days, and at the boundary of the solutions pentaphosphate crystals developed up to a size of 0.5-0.7 mm. The value of T for these crystals was 35-40 ~sec, but after annealing at 600°C the value of T increased to lO0-110 psec. The optical quality of these crystals was high. Other examples may be the crystals of CsNd tetrametaphosphate. The Cs and Nd compounds were added separately the phosphoric acid. The whole system was held at 320°C for 8-I0 days. Due to the counter-flow diffusion, very good crystals of CsNd tetrametaphosphate having = 120-135~ sec were obtained. Undoubtedly, from the point of view of obtaining high quality crystals for mini lasers, the counter-flow diffusion method is more useful, not only for the aqueous solutions, but also for the high temperature flux. This method was unfortunately not studied completely from the technological side. In addition to the (OH) ions, another important factor in the growth of crystals for mini-lasers is the existence of light-quenching admixtures like Fe, Ni, Co, Ir and Pt. The presence of Pt in the crystals was reported in the work (31). The authors propose an absolute remedy by replacing the platinum crucible with a

Table 3.

Nezoite

NdP5014

Influence of Hydroxyl Molecules on the Properties of Nezoites Temperature of crystallization

~

T

(°c)

(cm-b

(~sec)

320

45 - 50

30 - 32

0.09

30

400

n

55 - 60

0.17

450

15 - 20

65 - 70

0.20

500

8 - I0

80 - 90

0.25

550

3 -

5

If0

0.34

600

2 -

4

120

0.37

650

2 -

4

120

0.37

680

2 -

4

120

0.37

KNdP4012

350

-

ll5

0.41

CsNdP40 12

320

-

125

0.35

370

-

135

0.37

a-CsNd(PO3) 4

KNd(P03) 4

350

-

90

0.24

580

-

If0

0.30

300

-

65

0.23

400

-

500

-

75 -

II0

80

0.27

0.40

B. N. Litvln, K. Byrappa and L. G. Bebich vitreous carbon crucible. The transitional metals (Fe, Ni, Co) form octahedral and tetrahedral complexes in the phosphoric acid solutions, and these complexes according to their crystal chemical properties cannot go isomorphically into a dodecahedral nezoite complex. Therefore, these elements remain in the solution without entering the crystal lattice. The possibilities of their existence in the crystals as non-structural admixtures is less reliable. However, in the solution they form strong and comparatively big phosphoro-oxygen complexes, the existence of which in the crystal framework, according to the steriochemical understanding, is difficult. We added Fe (about 3 wt %) to the solution in the growth of Nd pentaphosphate and CsNd tetrametaphosphate crystals, but no visible changes were noticed in both the crystals as far as their laser characteristics are concerned. Since these admixtures have an ability to form stable complexes in the phosphoric acid solutions, in the growth of pure and admixtureless crystals greater preference should be given to the phosphates.

3.

CRYSTALLIZATIO~ BY THE FLUX METHOD

This method is also very popular in comparison with the earlier methods for obtaining monocrystals for miniature lasers (Table 2). In this method, the dissociation of Nd ions is often a result of surplus alkaline components, which should be presentin at least three times the quantity of Nd203 (Table 3). However, nezoites show a positive temperature coefficient of solubility during the crystallization from a flux. Therefore, the growth takes place by allowing a decrease in temperature according to a predetermined programme -- usually at the rate of l-2°C/hr (Table 2). We have considered the special features of obtaining menocrystals of phosphates by flux. The solution for obtaining the monocrystals of phosphates was prepared in the following way: the alkaline metals in the form of carbonate M2CO 3 were mixed with Nh4H2PO ~ and Nd203 in the molar ratio (3-6):(5-8):(I-2) respectively. The mixture was heated to 500-600°C and held for 8-16 hr. During this period, there takes place a reaction of the carbonate with NH~H2PO 4 to form a pyrophosphate M~P20?, polyphosphateMPO 3 or their mixture: M2CO 3 + 2 NH4H2PO ~ + 2

PO 3 + (CO 2

2 M2CO 3 + 2 NH~H2PO 4 ~ M 2 P 2 0 7

+ 2NH 3 + 3H20 )

+ (2C02 + 2NH 3 + 3H20)

since the temperature of melting of pyrophosphate is higher than the melting temperature of polyphosphates (Table 4), the ratio of M2CO 3 to NH~H2PO 4 should be such that the P205 content in the flux is not less than 40%. In this case, the crystallization can be carried out by increasing the temperature to 700-600°C. In flux growth, binary mixtures are mostly used, the suitable solvents being selected by using an empirical formula of Laudise (61)for the values of temperature melting and eutectic temperature. In Table 4 are given the most commonly used mixtures in the flux growth. We have tried most of these mixtures in the growth of Nd nezoltes. It was found that the growth is possible in all cases in the vitreous carbon crucibles, and this solves the problem of inclusions during the growth in the platinum, irrldium and other crucibles. The eutectlc temperature of most of these mixtures is nearly 700-600°C. Therefore, the temperature interval at which the growth takes place is usually 300-400°C (Table 2). When the rate of decrease in temperature is of the order of 0.5-1.5°C/hr, the growth process continues for few days. In this case it is somewhat difficult to stabilize the temperature in time.

Monocrystals for Miniature Lasers

Table 4.

Binary Solvents for the Growth of Nd Nezoite Crystals by Flux Method Temperature of melting (in °C)

Nezoite

Components of the solvent

first

second

Eutectic composition (in molar-Z) (first

component

component

component)

Eutectic temperature (°C)

Ref.

LiNd(P03)~*

Li4P207-LiP03

885

665

44.0

600

49

NaNd(P03) 4

NaPO3-Na4P207

627

960

61.0

540

50

Na3Nd(P04) 2

NabP207-NdF

998

627

66.0

734

51

KNd(P03)~*

K4P207-KPO 3

I090

809

49.5

590

52

RbNd(P03)4*

Rb4P2OT-RbP03

1000

805

22.0

602

53

a-CsNd(P03)4*

Cs4P207-CsP03

968

736

25.0

511

54

K3Nd(PO)2

K~P207-KF

1090

852

51.5

710

55

Na5Nd(WO~) 4

Na2WO4-Na2W207

686

738

60.0

610

56

Na3Nd(V04) 2

NaF-NdWO

995

632

17.5

596

57

LiNd(SO~) 2

PbSO~-Li2SO 4

868

855

22.5

632

58

K5Nd(MoO~) 4

K2MoO4-K2Mo207

925

506

49

460

59

Li2K5NdFI0*

KF-LiF

856

842

50

492

60

Crystals grown by the authors in the above mentioned binary systems.

In the flux method, we used the apparatus whose cross-section is shown in Fig. 5. The important features of this furnace are as follows: the external temperature insulation has been done with low temperature-conducting material (for example, sealing wax or fluorplast-4 (teflon). The heater was always at 800-700°C, and in the process of growth the temperature was held to an accuracy of I-2°C. The diamete~ of the heater should not be less than I0 times the diameter of the crucible. The higher this ratio is, the easier it is to obtain a minimal rate of decrease in temperature. This rate depends mostly upon solubility. If this data is lacking, as often occurs, the supersaturation has to be done from experience only. For this purpose, greater importance is given to the quantity of the spontaneous crystals developing from solvents of different volume at a constant cooling rate. More detailed information about the techniques of crystal growth by the flux method can be found in the works of Anthony and Kollong (62).

4.

PROPERTIES OF ND NEZOITES

The basic characteristics of the nezoites are: the number of Nd ions (N) per cm3; the shortest distance between Nd atoms in the structure; the fluorescent lifetime (T) of 4F3/2 Nd 3+ ions; fluorescent lifetime (T °) when z = 0.0l (concentration of Nd in the crystals); and the conditional quantum output. The enumerated characteristics of the Nd nezoites are given in Table 5. It can be seen in Table 5 that Li2K5NdFI0 shows a high quantum output and a high value of lifetime (T), but the existing technological difficulties do not permit its use practically. The other group of nezoites with a high value of quantum output (0.3-0.4), like NdP5OI4

268

B. N. Litvin, K. Byrappa and L. G. Bebich

v

v

9

T~

m

Fig. 5.

Crystallization chamber for the growth of Nd phosphate monocrystals by flux method: (I) ceramic stand (alumina); (2) temperature-isolation material (aluminia, fire clay); (3) heater; (4) crucible; (5) temperature-isolation screen (sealing wax, teflon); (6) asbestos; (7) mullite cotton; (8) crumbled aluminia; (9) ceramic glass (aluminia); (I0) ceramic bricks. TI, T 2 and T 3 Pt-Rh thermocouples.

and LiNd(PO3) 4 crystals, have already found practical importance in the development of the miniature lasers (2,3,6). Taking into account their good characteristics for practical applications, we can conclude that this group of nezoites will be of great technological importance. Table 5.

Nezoite

~rad.

Properties of the Crystalline Nezoites

N1021

Shortest Nd-Nd distance

x (~sec) ± I0 ~ z

zo

Ref.

NdPsO14

1.051

3.89

5.19

120

320

0.38

7,

19

NaTaTO19*

1.060

3.10

-

120

275

0.44

8

KNdP4012*

1.050

4.06

6.59

I10

280

0.39

a

RbNdP4012

!.050

4.09

6.129

100

290

0.34

a

CsNdP4012*

1.049

3.93

6.70

i15

360

0.32

a

LiNd(PO3) 4

1.047

4.36

5.62

II0

320

0.34

13, 4

NaNd(P03) 4

1.051

4.24

5.72

53

260

0.20

14, a

KNd(P03) 4

1.052

4.08

6.66

I00

275

0.36

15

RbNd(PO )

1.050

4.01

6.13

90

280

0.32

a

Monocrystals

for Miniature Lasers

269

Table 5 (Continued)

Nezoite

Irad.

~021

Shortest Nd-Nd distance

T (psec)± I0 q T

To

Ref.

s-CsNd(PO3) 4

1.050

4.70

5.76

]I0

370

0.30

a

B-CsNd(PO3) 4

1.050

3.50

6.69

115

360

0.32

a

Na3Nd(VO 4)

l 048

2.60

-

30

320

0.09

a

Na3Nd(PO4)2

.050

2.60

5.72

23

359

0.064

17, 63

K3Nd(P04)2

.055

5.00

4.87

21

458

0.046

21, 63

K5Nd(Mo)4)4

.066

2.37

5.96

70

215

0.320

64

AI3Nd(BO3)4

.065

5.43

5.92

19

50

0.380

23, 39

Li2K5NdFI0*

.048

3.60

6.72

300

500

0.600

24, 65

LiNd(S04)2*

.050

52

265

0.190

a

structures are not interpreted. a This paper REFERENCES I.

H.G. Danielmeyer, G. Huber, W.W. Kruhler and J.F. Jesser, Appl. Phys. ~, 335 (1973).

2.

H.P. Weber, T.C. Damen, H.G. Danielmeyer and B.C. Tofield, App,. phys. Lett. 22, 534 (1973).

3.

H.P. Weber, P.F. Liao, B.C. Tofield and P.M. Bridenbaugh, Appl. phys. Lett. 26, 692 (1975).

4.

K. Otsuka, T. Yamada, M. Saruwatari and T. Kimura, JEEEj J. quGnt. Electr. qE - I I , 330 (1979).

5.

K. Kubodera and K. Otsuka, J. appl. Optics, 18, 884 (1979).

6.

K. Kubodera, K. Otsuka and S. Miyazawa, App/. Opt~sj 18, 884 (1979).

7.

K.R. Albrand, R. Attig, J. Fenner, J.P. Jesser and D. Mootz, Mat. res. Bull. ~, 129 (1974).

8.

Jean-Claude Michel, D. Morih, J. Primot and F. Auzel, C.R.Acad. Sci. Paris, B,

284, 555 (1977). 9.

Par Rene Masse, Jean-Claude Guitter and Andre Durif, Aeta Grystallogr. B, 3_~3,

630 (1977). I0.

K.K. Palina, V.G. Kuznestov, N.N. Chudinova and N.T. Chibiskova, Inorg.

Materials, 12, 730 (1976). II.

H. Koizumi and J. Nakano, Acta Crystallogr. 33, 2680 (1977).

12.

B.N. Litvin and K. Byrappa, ECCG - 2, Poster paper, Lancaster, England, 10-15 September (1979.

i3.

H. Koizumi, Acta CzvjstalZogr. B, 32, 266 (1976).

14.

H. Koizumi, Acta Crystallogr. B, 32, 2254 (1976).

15.

K.K. Palkina, V.G. Kuznestov, N.N. Chudinova, N.T. Chibiskova, DokZ. Acad. Soi. USSRm 226, No. 2 (1976).

270

B.N.

Litvin, K. Byrappa and L. G. Bebich

16.

S.I. Maksimova, K.K. Palkina, V.B. Loshenov and V.G. Kuznestov, J. inorg. Chem. 23, No. 11 (1978).

17.

R. Salmon, C. Parent, M. Vlasse and G. Le Flem, Mat. res. Bull, 39, (BI3) 444 (1978).

18.

M. V l a s s e , I n o r g . Chem. 1 5 ,

19.

H.Y.P. Hong and K. Dwight, Mat. r e s . B u l l . 9, 775 (1974).

20.

B. J o u k o f f , M. F a d l y , J . O s t o r e r o and H. Makram, J. c r y s t . Growth, 4 3 , 81 (1978)

21.

H.Y.P. Hong and S.R. Chinn, Mat. r e s . B u l l . 1 1 , 428 (1976).

22.

P.V. Klevstov, L.P. Kozeeva, V.I. Protasova, L.Yo. Harchenko, L.A. Glinokaya, P.F. Klevstova and V.V. Bakakin, CPystallogr~rphiu, 20, No.l (1975).

23.

H.Y.P. Hong, Mat. res. Bull. 9, 1661 (1974).

24.

H.Y.P. Hong, Mat, res. Bull. 14, 137 (1979).

25.

B. Blanzat, J.P. Denis and J. Zoriers, Pmoc. Xth R ~ 1170 (1973).

26.

B.C. Tofield, H.P. Weber and T.C. Damen, Mat. rs8. Bull.9, 435 (1974).

27.

H.G. Danielmeyer, Festko~perprobleme 253 (1975).

28.

H.Y.P. Hong, Aota Crystallogr. B, 30, 468 (1974).

29.

A.V. Chlganov and N.V. Belov. C~ochem~stryj 12,

30.

D.C. Miller, L.K. Shick and C.D. Brandle, J. Üz,jst. Growth, 23 313 (1974).

31.

H.G. Danielmeyer, J.P. Jesser, E. Schouherr and W. STetter, J. tryst. Growth, 2 2 , 298 (1974).

32.

B.C. T o f i e l d ,

33.

Hiroyuki Kasano and Yoshio Furuhata, J. Electrochem. Soc., Solid State Science

(6) 1440 (1976).

Earth Conf., Arizona,

(Advances in solid state Physics),

15,

1456 (1968).

P.M. Bridenbaugh and H.P. Weber, Mat.res. B u l l . I O ,

1091 (1975).

& Teahno~ogy, 126, 1567 (1979). 34.

T.R. Au Coin, A. Schwartzm H,G, Gualtieri and M.J. Wade, U.S. Patent 4,043,860 (1977).

35.

J. Nakano, T. Yamada and S. Miyazawa, J. Amer. Cerc~. SoÜ. 62, 465 (1979).

36.

T. Auzel, Mat. res. Bull. B, 14, 223 (1979).

37.

J. Nakano, K. Otsuka and T. Yamada, J. eppl. Phys. 47, 2749 (1976).

38.

N.N. Chudinova and N.V. Vinogradova, Inorg. Mater~I8,

39.

S.R. Chin and H.Y.P. Hong, Opt. Commun. 15, 345 (1975).

40.

A.A. Balman, Amer. Min. 47, 1380 (1962).

41.

H.Y.P. Hong and K. Dwight, Mat. r e s . B u l l . B, 9,

42.

II (1975).

1661 (1974).

G. T a r b u t t o n and M.E. Detaining, J. Amer. Chem. Soc. 7 2 , 2086 (1950).

43.

E.H. Brown and C.D. W h i r r , Ind. engng. Chem. 4 4 , 615 (1952).

44.

N.N. CHudinova, L . P . S c h o l o v e r and G.M. B a l a g i n a , Inorg. M a t e ~ 1 8 , (1975).

45.

H.K. Henisch, C ~ s t a l Growth in C~18, Pennsylvania State University Press, University Park (1970).

46.

B.C. Tofield, H.P. Weber, T.C. Damen and P.F. Lia, J. solid st. Chem. 12, 207 (1975).

47.

B.C. Tofield, H.P. Weber, T.C. Damen and C.A. Pasteur, Mat. res. Bull. 9, 435 (1974).

]._[I, 686

Monoerystals 48.

for Miniature Lasers

S.R. Chinn and H.Y.P. Hong, Appl. phys. Left. 26, (II) 649 (1975).

49.

J. Nakano and T. Yamada, J. Amer. Ceram. Soc. 5_~9, 172 (1976).

50.

A.G. Bergman and V.A. Matrosov, J. inorg. Chem. 14, 1669 (1969).

51.

V.P. Goryacheva, A.G. Bergman and A.I. Kislova, J. inorg. Chem. 4, 2744 (1959)

52.

A.G. Bergman and Z.I. Sanzirov, J. inorg. C~m.

53.

V.A. Matrosov and A.G. Bergman, J. inorg. Chem. 2__4, 2279 (1979).

54.

E.L. Krivovyazov, V.P. Balkova and N.K. Vorkresenskaya, 761 (1970).

55.

A.G. Bergman, K.F. Kaznacheev, V.P. Goryacheva and A.P. Sadovekii, J. inorg. Chem. 8, 1455 (1963).

56.

P.I.

15, 1139 (1970).

Inorg. Materials, 6,

F e d o r o v and M.V. Mohosoev, J. inorg. Chem. 6, 243 (1961).

57.

M.A. Zaharchenko and A.G. Bergman, J. inorg. Chem. ~, 877 (1957).

58.

D.S. Lesnich and A.G. Bergman, Scientists' Notes of Rastovskii University, 2__O0, 19 (1954).

59.

V . I . S n i s t i n and I.M. K u l e s h o v , J . gen. Chem. 2 ! ,

1367 (1951).

60.

S . I . B e r e z i n , A.G. Bergman and E.L. Bakumskii, J. ino~J. Chem. 8, 2140 (1963).

61.

R.A. Laudise, The Growth of Single Crystals, Prentice-Hall, Englewood Cliffs, New Jersey (1970).

62.

A.M. Anthony and J. Kollong, Preparative Methods in Solid State Chemistry, Academic Press, New York (1972).

63.

S.R. Chinn and H.Y.P. Hong, Opt. Electr. 188, 87 (1976).

64.

A.A. K a n i n s k i i ,

S.E. S a r k i s o v , I . Bohm, P. R e i c h e , D. S c h u l t z e and R. V e c h e r , (a) 4_/3, 71 (1977).

Phys. Star. Sol. 65.

A. L e m p i c k i , B.C. McCollum and S.R. Chinn, JEEE J. quint. Electr. QE 14, 283 (1978).