There is no specific format for a .dat file. You can use writematrix(): https://www.mathworks.com/help/releases/R2020a/matlab/ref/writematrix.html or dlmwrite(): https://www.mathworks.com/help/releases/R2020a/matlab/ref/dlmwrite.html in older versions.
For example,
writematrix(sr, 'filename.dat') % or dlmwrite('filename.dat', sr);